This is a nasty little issue. On my private network, suddenly some endpoints would refuse to establish calls made over SIP or H323. Error codes would be:
channel unacceptable 487 / Request Terminated 480 / Temporarily Not Available
Not particularly helpful information. After all the packet sniffing and diagnostic checks, it appeared packets were being rewritten or dropped between the endpoint and our VCS. The culprit here is Juniper's 'ALG' for SIP and H323 traffic. I don't have the time not to figure out exactly why or how it was breaking my calls, but i do know that in my case, it wasn't needed. Disable the SIP and H323 ALG's (in the SRX web UI under Configure -> security -> ALG. The endpoints will start working again after your reboot them and allow them to re-register to your gatekeeper.