Mixing SIP with PRI circuit for trunking

[IT Hamster]

Hey all,

Have any of you combined a SIP Trunking service alongside a PRI Circuit through a gateway?

We're looking to get a PRI circuit to handle 23 channels for 23 DIDs and then excess DIDs would then be trunked inbound over a SIP Trunk.

I've done one and the other, but never both in the same PBX. Thoughts on this?

 

[leejor]

I would assume that these would simply be seen by 3CX (set-up) as two different trunk groups, and routing would have to be set up to reflect that. I don't imagine that you are subscribing to the same service provider for both, in which case certain incoming numbers will be relegated to a certain trunk. You can divide, or route outgoing calls as needed.

 

[lneblett]

It is not an issue.

I do have an observation however -

There is no reason that the PRI cannot handle more than 23 DIDs unless the provider limits it. It is fundamentally 23 call channels that can process DIDs as needed. It's really a question of how many external calls do you expect to have at any one time and whether the PBX is licensed to support it (along with the internal calls)?

You did not state what the overflow would do, but you might be able use only PRI or only SIP to meet the need. Of course, there may be other reasons for the combination and that is fine. Just wanted you to know that the need for SIP may not exist as the PRI might be enough.

 

[IT Hamster]

Thanks for that input!

These would be two different providers. The main reason we're doing this is that our SIP Trunking is subjected to WAN outages.

These outages impact services for customers and even having 1 brief outage over a 6 month period yields threats to contracts and all that fun stuff. Essentially, I'm trying to find a way to get the stability of a PRI circuit for trunking, but I was going to keep the SIP trunk since I wasn't sure how many DIDs could be supported on a PRI- I've got 30+ DIDs currently for some clients.

The PBX is licensed for 32 simultaneous calls, all of which may be used at some point- I have yet to figure out if 3CX provides metrics for "maximum simultaneous calls in any given period" so I could better determine demand for concurrent calls.

If the PRI Trunk only supports 23 calls I may need to figure out how to make the overflow use SIP trunking.

They would indeed be two separate trunks, though the majority of their call volume comes in over one inbound DID (Their Toll-Free) which would probably be ported to the PRI or forwarded on from the SIP carrier.

 

[lneblett]

As far as I know, 3CX does not provide info about max call usage other than to send an email, if set-up, and the max is hit. There are some 3rd party apps that purport to do this.
https://www.voiptools.com/3cx-call-counter-by-voiptools

Is one and I think a couple of others exist as well.

Calls are counted against the max when calling from extension to extension and in some cases may take 2 licenses when forwarding is in effect. If you had all 32 in use to external numbers (outbound or inbound), then no other calls will be allowed.

The overflow is handled by outbound routes. If the PRI will not accept, then the secondary route to SIP will kick in. You will need to give this some thought as every outbound call going via PRI is also one less call that can come in on the PRI. Your CID management will also need to be examined as callers will receive this and likely use for all future callbacks. Check with the SIP provider to insure that he allows or can set CID manipulation.