When the use of SIP trunking was initiated (pun intended) to overcome the inflexibility of traditional PRI trunks with fixed channel limits and costly rate plans.
The big picture view of SIP is great; SIP enables VoIP, Unified Communications (UC), enhanced features, and more.
In a perfect world SIP trunks would be plug-and-play but, compatibility issues and interoperability are still a challenge.
When SIP isn’t SIP
In 1996 Session Initiation Protocol (SIP) was designed by Mark Handley, Henning Schulzrinne, Eve Schooler and Jonathan Rosenberg. A few year later in 1999 the SIP protocol was standardized as RFC 2543.
The creation of SIP provides the standard on which SIP trunking operates.
The SIP standard is open to interpretation, at least that’s how it’s been viewed by telephony hardware manufacturers.
The issue with SIP is best explained by Andrew Prokop in his article, Getting SIP Right the First Time Around:
SIP is a standard that is managed and enhanced by the Internet Engineering Taskforce (IETF), but like many standards, people have taken liberties with how they’ve chosen to implement SIP.
This means that, theoretically, a SIP trunk call won’t go through you’re using an Avaya SIP phone and you’re trying to call someone on the other end who is using a Grandstream SIP phone.
With a little planning two similar but different SIP communication networks can be bridged together by keeping these SIP trunk deployment considerations in mind.
SIP Trunking Provider
What platforms are compatible with your SIP trunking provider?
SIPTRUNK.com for example has been certified and field tested to work with IP-PBXs from:
You should also be able to find SIP trunk configuration guides from your provider to ensure interoperability and a smooth set up.
Does your current internet connection have enough bandwidth to handle VoIP calls?
High speed broadband internet and a dedicated portion of the network is a must to send and receive calls over a SIP trunk. You can do a simple internet speed test to determine if SIP trunking is compatible with your existing PBX.
Do you have a hosted or, on-site PBX?
Hosted PBXs are managed off-site by a VoIP provider whereas the on-site PBX is maintained by your internal IT staff. Regardless, make sure the PBX is interoperable with your service provider.
Are VoIP gateways needed to convert PBX to an IP-PBX or, is a Session Border Controller (SBC) necessary for transcoding?
SIP trunks can be adapted to work with a variety of systems but, some additional hardware may be needed to complete the connection.
A lot of hardware and software is advertised as being “standards based” but it’s unlikely that you’ll be purchasing all the necessary components of a communications system from the same manufacturer, which is why you have to spend a little time making sure all these pieces play nice together.
SIP trunking is not impossible, it just takes some planning, adapters, and maybe some transcoding.
Every day millions of phone numbers are dialed and calls are bridged – we have the technology.
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