SIP trunks are a relatively new way to bring voice traffic into your company.  These are trunks that are delivered through the internet or some other form of IP traffic.  This is a move away from traditional T1s that were the standard to bring multiple lines of traffic into a location.  So why the change?  Well, IP is now at the center of communications.  SIP trunks do offer some nice features, but be careful when evaluating these trunks as replacements for your T1s and make sure you have the right reasons to make the switch.  Below are some pros and cons that I have come across in my experience.


One of the biggest factors that we had in moving to SIP trunks was portability issues.  Say you have an office in Florida with a phone system and one or many T1s terminating there with 100s of local numbers.  Here is a problem unless you have another Florida office with a phone system and capacity to handle the additional circuits.  The main problem with traditional T1 service and loca providers is these numbers are locked into Florida and must either terminate to equipment in Florida or be call forwarded which can run upwards of $.07/minute.  If you run a call center with lots of calls, this can add up quickly to thousands of dollars rather quickly.  SIP allows you to terminate these numbers to a provider who can then deliver them anywhere in the world over an IP network for the same rates you were used to or less.  This is a good thing and a big money saver.

SIP trunks are also easily scale-able.  In the past with T1s, you had 23 trunks per line, or 23 call paths per T1.  when you needed more, you ordered another T1.  You also had to have another interface as well as space in your PBX to handle the card to interface the T1.  SIP trunks scale much easier with simply adding bandwidth and sessions, the virtual equivalent to  trunks in a T1.  The interface stays the same, an ethernet connection, and this can usually be done on the fly or with minimal downtime which is great.


Up front expense.  We interfaced with an Avaya PBX, and the equipment required to sit between the MPLS cloud and the phone system was expensive.  Avaya, much like other vendors, required a proprietary Avaya session controller which, well, controlled the SIP sessions.  We also required a border controller which is where the external IP address that talked to the SIP provider resided.  We ran this solution over our existing MPLS network to allow for QOS and other prioritizing features, so the bandwidth to support voice needed to be upgraded which is also an added expense.  We were looking to offset the previously mentioned call forwarding charges, so there was quite a bit of costs that we were offsetting which still made it viable for us, but this all needs to be taken into account.

One other big con that we ran into was the issue of a SIP trunk outage.  We found out the hard way, that the numbers routed to the SIP trunk were basically stuck there in the case of an outage.  We only had one location with SIP service and we later found out that the numbers on SIP can only be routed to other SIP trunks if there was a problem at the primary site.  This applied to all local numbers that were being sent to the SIP trunks.  So even though we had other T1 service in our account, our numbers were stuck in limbo until the issue was resolved.  The solution?  Double everything mentioned above and replicate this service at another site.  If DR is a priority for you, this is the only way to obtain it.

I hope this has given you some insight into the world of SIP trunking.  It is a bit of a complex solution and it has it’s little quirks, but in the right situation, it can be a huge cost saver in the long run.

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