What is the key to FoIP success when using SIP trunking? First, you must have FoIP endpoints that really work. For the highest success rates they must support both G.711 pass-through and T.38 since not all calls will receive a T.38 re-Invite quickly enough or at all. And, if the SIP call-control logic doesn’t correctly handle those late re-Invites, you’ll have to accept an increased failure rate regardless of what you do. But, after that, it’s up to you and your ITSP (Internet telephony service provider) to make sure your calls are being correctly handled. And, just because they are today, don’t count on their being effectively handled tomorrow.
Below is an online chat (edited to obscure the participants) between a very knowledgeable FoIP-based fax broadcaster and his very knowledgeable and helpful ITSP. It’s in reverse chronological order, so skip to the end and read back to here.
This type of involvement is required today because we are in the midst of the transition from TDM to IP networks, and you often have a mixture of the two. So the route your service provider and his carrier partners use to get to the endpoint terminal is all important. And if they don’t employ FoIP-specific routing, your chances of a successful call go way down.
So, if you want carrier-based FoIP to work for you, use fax endpoint products that work and use service providers that are experienced with FoIP and willing to work with you.
Begin chat. Actually, this is the end of the chat, so read it backwards.
>John Smith
>Fred,
>We only have one route for this destination - if that route is at capacity your call will be rejected.
>----------------------------------------------
>John,
>Testing shows 70% failure with 11 calls.
>Here are both-side PCAP files. Call me if you need real-time testing.
----------------------------------------------
>John Smith
> Hey Fred,
> I'm able to complete the call over Big IP now. Can you try again?
> -------------------------------------------------------
> John Smith
> Hi Fred,
> I'll need to look - I wasn't pulling any BIG IP routes
> for this number which might mean that they don't
> terminate to that area.
> -------------------------------------------------------
> Fred,
> How is this resolved?
> Can you route via Big IP?
> They seem to be the best.
> -------------------------------------------------------
> John Smith,
> Fred,
> Looks like Legacy Bell is our only route for this number.
> It'll complete some of the time - we're trying to
> increase capacity with them, though. Some of the
> rejects are simply due to capacity.
> -------------------------------------------------------
> John Smith,
> Fred,
> Wasn't T.38 in this case – Legacy Bell, actually. Still
> trying to see if this was/is the only available
> vendor for this area.
> -------------------------------------------------------
> Fred,
> On 3/8/2012 9:53 AM, John Smith wrote:
> One vendor is fine as long as it is Big IP.
> They seem to be the only reliable T.38 vendor.
> -------------------------------------------------------
> John Smith,
> Hi Fred,
> Sorry for the delay - getting a new environment up
> and running and just got Wireshark installed. I see
> all the calls reaching the same vendor, but it
> appears we only have one route available for the call
> on the T.38 deck. I'm trying to confirm this, since
> the number is near Atlanta I would assume we have
> more routes. I'll update you shortly.
>.-------------------------------------------------------
> Fred,
> The Xxxxxx system seems not to be able to accept and
> place calls. see attached PCAP file
> --
> Fred
Edited by Juliana Kenny