Call Transfer Problem

[btamiletisim]

We have a problem with call transfer. 

When a user transfer an internal call to an internal directory number everything works fine. But, when an external call transferred to an internal directory number sometimes it succeeds and sometimes it fails. 

We use cucm 12.5 and the cisco 4451 router as session border controller. The structure is simple:

phone---CUCM---SBC---ITSP

What would be the reason? Where does it make sense to search and troubleshoot? I am not an expert. Could you help and guide me about the situation?

Thanks.

[Sadav Ansari]

Means when external caller dial the IP phone number and ip phone transfer the call to another ip phone it’s intermittent issue right ?

Collect the CCM log and debug from the Voice gateway and check from which end call is disconnecting or from where disconnect/BYE coming, Also check the region of disconnect of the call .

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[btamiletisim]

Thank you for reply. I will try to make a problematic call transfer and collect logs. I want to ask some questions:

1. You mean SDL logs by saying CCM log, right?
2. Which debug commands would you suggest to run on voice gateway?
3. Where can I "check the region of disconnect of the call"?