A SIP Trunk for Lync, Sir?

At time of writing this Game of Thrones Season 5 is still about six months away! Random comments aside, and on a more Lync related note…SIP Trunks. The pre-engagement process is a critical part of any Microsoft Lync deployment. Regardless of the size, scope or purpose of a potential Lync project, any consultant worth their salt will always engage with their client fully in this regard. It’s purpose is to ascertain every single detail that might influence the proposed solution, implementation, maintenance, and on-going functionality of the Lync platform. Over time I’ve found myself referencing the same template of questions again and again; everything from preferred certificate vendors through to thin client hardware specifications. An area that often results in further discussion is existing PSTN connectivity;

“Could you please advise how your current telephony system is connected to the PSTN (eg. ISDN30, SIP Trunk etc.)?”

A typical response might be that the client has a single PRI ISDN connection, at which point I might ask them about any future intentions to migrate away from ISDN to a SIP Trunk Service. Perhaps this is something they’d like to consider as a logical follow on or complimentary project to their Lync roll out.

“Away from ISDN to a what?” …now depending on whether I’m already on my way out the door, I might take the time to explain this to them myself, or alternately provide enough additional reading material to bring themselves up to speed. Once I’ve finished writing this I guess I’ll be able to reference my own post! POW! POW! *smoking guns*

There are plenty of ‘What is a SIP trunk’ articles out there, and here’s another – purely from the Lync perspective of replacing your traditional ISDN services with a SIP trunk.


Keeping it very simple, a SIP trunk is an alternative to ISDN for connecting your Lync environment to the PSTN (Public Switched Telephone Network). Unlike an ISDN connection whose calls will traverse dedicated physical wires and cables that terminate on your premises, a SIP trunk will use an existing (or new if desired) data network connection such as broadband or a leased line and access the PSTN over IP instead. Ultimately providing you with the same service you would get from a traditional phone line and enabling your Lync users to place and recieve external calls. A SIP trunk from an ITSP provider can be connected directly to a Lync mediation server or to an SBC (Session Border Controller).


The most common business driver behind a migration to a SIP Trunk from ISDN is costing saving, I’ve never heard of a SIP trunk service costing more than the ISDN service it replaced. Statistics can be manipulated to prove anything, and depending on what you read and who you speak to, companies have been known to reduce total cost of ownership for PSTN connectivity by anything from 20-60%. Numbers aside there is almost always a saving to be made, just how much of a saving is obviously environment and company specific. Below are just some of the underlying reasons that result in this financial gain, as well as other benefits that make a SIP trunk solution desirable.

Administrative Flexibility
Increasing the number of concurrent calls (channels) or adding new DDI’s to your number range is a ‘thumb twiddling’ process from an ISDN perspective. SIP providers in general are capable of making these types of changes in a comparatively much quicker time frame. Some SIP providers will sell the fact that they can make these changes within 24 hours, and depending on your contract this might also include other tasks such as number redirection and manipulation. Other providers also accommodate flexibility in terms on channel limitations; perhaps you have the ability to make 10 concurrent calls on your SIP trunk, but happen to place 11 calls one day. A one off? that’s fine, but make it a habit and you’ll be asked to upscale the number of channels you’re paying for. Now although all the above is completely dependant on the SIP trunk provider – know that the flexibility is there.

Subscriptions & General Cost
DDI’s, Channels, and SIP trunk service costs in general are typically less, and work out particularly well for you if you fall into particular scenarios; In the UK a PRI ISDN connection will afford you 30 concurrent calls (channels). You might pay for all 30 of those channels if you want to make 30 concurrent calls. So what if I want 40 concurrent calls? Well I have to pay for another PRI even though I only want another 10 channels. This problem goes away with SIP trunks where we can scale and increment the number of channels as needed, we’d simply ask for another 10 and pay that cost.

The same is true at the opposite end of the spectrum for a company with lesser requirements. If you only want 6 SIP channels, then pay for 6. The minimum number of PRI channels is 8, so you wouldn’t be able to achieve this on traditional PRI, instead you’d be one of those places with 3 X BRI boxes stuck on the wall (BRI offers you 2 channels). But then what if you’re requirements grew?… you get the point. Once again this comes back in part to being a very flexible service.

Equipment & Maintenance
As a SIP trunk service is going to leverage the IP network, there’s no requirement to have bundles of telephony cables and wires coming into your premises. Getting rid of these also does away with any maintenance or administrative costs associated with them. Strictly speaking there is no hardware requirement for your new SIP trunk either, although an SBC should be used as per industry best practice which I’ll touch on shortly. You’d be left with a consolidated IP network for both data and voice… much better.

Disaster Recovery
Perhaps this also comes under flexibility, but one of the few things your SIP trunk provider will need to know to send calls your way is the public IP address that you wish to receive them on. From a Disaster Recovery perspective this affords you the ability to redirect hundreds of phones call or DDI’s simply by specifying an alternate IP address in a time of need. This isn’t a DR exercise and there will be plenty of ways to skin the cat with the assistance of the provider, but once again – know that this kind of flexibility far exceeds that of existing ISDN services.

Additional Side-line Benefits
Might include things such as a reduction in long distance call charges, depending on your provider and contract type. Some SIP trunk providers will also make call detail records available via a web based portal system for review, and although we can parse this type of information from our own Lync databases its always useful to have expansive options when it comes to call billing and analytics etc. Voice quality will be nothing less than equal to your existing ISDN service, and if encryption of traffic is required then there are also providers that will offer plans for the security conscious.


It’s the next logical question. If SIP trunks are so great, then why hasn’t everyone jumped ship? Well statistically they are. SIP trunk adoption is growing rapidly year on year, but naturally it’s never a clear cut decision and there are a couple common issues that might obstruct your business justification to migrate.

The continued expansion of high speed internet access across the globe is one of the main reasons SIP trunk adoption rates are climbing, but if you reside or operate from a poorly connected area then this might pose a problem for you. You should be able to provide the bandwidth necessary to run both your normal data workloads and voice workloads – and comfortably. Don’t attempt to do any grand mathematics on your own, let your ISP / ITSP make both the proposals and the promises, but in short you’ll need sufficient bandwidth to accommodate your SIP trunk.

Existing ISDN contractual agreements are another reason that often puts SIP trunk migration on the back burner. If your tied in on contract, and the money you would save is not going to exceed the amount you’d sink by jumping out of contract, then obviously it’s savvy to stay put for the time being.

There may also be other sector specific requirements that prevent an immediate move. Different governments operate to different standards, do these standards extend to SIP trunk services and whether any potential trunk should be of a certain qualified criteria? Perhaps, after all we’re on an IP network now. The same might apply to top level financial or defence corporations. I’ve no doubt that there are other road blocks I haven’t listed.

OK, SIP Trunk… I Want One

There are a lot of SIP trunk providers out there, and from a Lync perspective they’ll fall into one of two categories: ‘Qualified for Lync’ or ‘Not Qualified for Lync’. A complete list of Microsoft Lync qualified providers can be found here: http://technet.microsoft.com/en-us/office/dn788947.aspx

So lets clear up what this means when choosing a SIP trunk provider. A Lync Qualified SIP trunk provider has had their SIP trunking service tested by Microsoft against a Lync environment. The SIP trunk has been directly terminated on a Lync mediation server, the provider has modified the service to work natively with Lync, and passed all the vigorous testing that Microsoft perform as part of their interoperability program. These trunks are proven to work, and if you ever logged a fault with the provider the engineer would likely know of Microsoft Lync as a Telephony (UC) solution.

In contrast, a non-Qualified SIP trunk has not been tested against those same interoperability criteria, but this doesn’t mean that you can’t use it. It does mean that terminating it directly on a Lync Mediation server is not supported. If you use a non-Lync qualified SIP trunk then you should use an SBC (Session Border Controller) to terminate the SIP trunk, and then pass it to your Lync environment from there. Non-qualified SIP trunks might be passing messages or requesting features that Lync doesn’t support, the SBC can be used to correct this before passing them to the Lync environment. This is a very basic high level interpretation, and this isn’t an SBC article. You’ll find that a Lync qualified trunk will typically cost slightly more as well.

It should be noted that my personal stance as shared with many is that you should ALWAYS deploy an SBC when using a SIP trunk regardless of Lync qualification status.

SIP Trunk

If you feel like dipping your tow in the water with SIP trunks, some of the more reputable providers will also offer trial services. PureIP for example will provide you with a free 14 day trial of their services, along with a handful of DDI’s for testing purposes. Their trunks are qualified for use on Lync, so you could test the quality of their service for an interim period without the need to invest in an SBC – handy.

On making the jump to a SIP trunk, migration of all your existing telephone numbers (DDI range) is achieved through a number porting process. This process is taken care of between the old and new providers, but can be rather a rather lengthy one. If you’re staying with the same provider, but simply moving from their ISDN service to the SIP trunking one, then this time frame is normally shortened considerably.


In todays well connected world you’d be naïve not to ask yourself whether it’s worth considering a migration from an ISDN service to a SIP trunk. It is genuinely cheaper for a carrier to provide PSTN connectivity through a SIP trunk than ISDN, and these savings are being passed directly on to the subscribers resulting in a cheaper service for us. The question should be asked, “Why aren’t we migrating to a SIP trunk?” – if you have an answer to that question then that’s fair enough. If you don’t, then start building your business case!

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