No matching peer over SIP Trunk

I have a Grandstream UCM PBX with a SIP trunk to an OpenVox GSM Gateway, at the SIP Logs I see the call coming through the trunk but I get this message:

No matching peer for '5900-10.11.12.10' from '10.11.12.10:5060'
user_and_ip=5900-10.11.12.10, res=-100, sipmethod=5.
peer=(null).
peer=(null).
No matching peer for '5900' from '10.11.12.10:5060'

5900 is my extension, 10.11.12.10 is the UCM, the call fails as apparently OpenVox is not finding a way to return the call. The trunk is anonymous and none as registration.

Hi,

I need to check it for you remotely so that i can know more details.
Please contact me directly when you available,my contact info be shown as below:

Skype: rick.zhu3
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Best Regards.