We are trying to track down a problem where we sometimes have dropped calls between our main office and a remote office.
The system is a UCM6510 with 126.96.36.199 firmware. Both ends that experience the dropped calls are Grandstream GXP2170 phones.
The topology looks like this: extension 1 (remote office) -> VPN -> UCM6510 (local office) -> extension 2.
The firewall that handles the VPN does not have any services enabled that would change the packets (filters, etc). All ports and traffic is allowed to go between the sites.
When extension 1 calls extension 2, it rings until it is picked up, and which point the call drops. Both sides hear a dial tone. They both hang up, at which point extension 1 can call back immediately and it works fine.
We turned on syslogging on the UCM but it comes though kind of like a normal hang up, there’s no SIP errors that are generated. We could capture packets but it’s not reproducible on demand, we would have to run a packet capture for a few days in a row before it happens again.
What would the recommended settings for extension 1 be? Any suggestions would be appreciated.