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Transfering calls that originate from SIP trunk drop

February 11, 2010 4:05 PM PST
Justin Yaple
First Community CU (SOFCU)

I have setup a SIP trunk group with two SIP trunks to Gafachi for LD/1-800 service. Iv got outbound LD, and inbound 1-800 working, but when an inbound 1-800 call is transferred either manually, by a workgroup, or by an auto-attendant the call drops. The call never terminates from the callers end it just goes dead, but remains connected.

The initial call does come in, and reachs the trunk group destination. I have set the destination to my own extension, a workgroup, and an auto-attendant all of them the call rang, and was answered.

When trying to manually transfer the call from my extension to someone else the PCM displayed an error "Complete Transfer: The operation failed for unspecified reasons".

ShoreTel support says its an unsupported provider to I need professional services to get it setup, but everything is already setup because I am able to place outbound LD calls, and receive inbound 1-800 calls. Its after the call is received that the ShoreTel system is dropping the call.
February 12, 2010 2:42 PM PST
Amanda Fagan
NB Test and Measurement
Are you using an InGate as your SIP Session Border Controller? If not, you will have a very hard time getting it to work (if at all) because the ShoreTel system assumes all SIP endpoints are on the same network with no firewalls in between them.

When you transfer a SIP call, ShoreTel sends a message to the other end telling it to stop sending audio to the current destination (aka your phone), and instead to start sending audio to a new destination (aka the other phone). The problem is, ShoreTel only knows about your "internal" IP addresses which your ITSP does not have access to. In effect ShoreTel is telling the other end to start transmitting audio to an unroutable IP address. The job of the InGate (or a Session Border Controller) is to intercept these packets and re-write them to use the "external" IP address of your firewall instead.
February 12, 2010 5:59 PM PST
Justin Yaple
First Community CU (SOFCU)
No, I am not using an InGate SIP Session Border Controller. My firewall is capable of doing exactly what you describe though. I was talking with a ShoreTel engineer, and they said if the ShoreTel system receives any SIP messages that it does not know how to process due to variables in the SIP messaging protocol it can cause these issues.

I am going to setup a test with one of the certified ShoreTel providers to see if it will work with them. Just to verify if its a SIP configuration issue between ShoreTel and the provider, and not something to do with using the firewall as the SIP Session Border Controller.
February 16, 2010 11:51 AM PST
Amanda Fagan
NB Test and Measurement
I think you are on the right track. A Session Border Controller does more than re-point voice packets, it also needs to re-write those other variables in the control messages as well. The InGate is just pre-programmed to do that for ShoreTel systems.
March 09, 2010 11:27 AM PST
Juan Rubio
Justin, not sure if you're still having issues, but without an Ethernet capture I'm only guessing as to what could be the problem... By default the ShoreTel system will use REFER messages to transfer calls, if the ITSP does not support REFER messages transfers will fail. The Ingate SIParator handles REFER to re-INVITE conversions for us, so it does a bit more than what an ALG firewall would do. If you're using ShoreTel 9.x (or greater) you can try to change the SIP trunk profile from _SystemTrunk to _ATTBVOIP. The _ATTBVOIP SIP trunk profile does not utilize REFER messages. Good luck.

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