Randomly getting silent calls

It’s happening almost daily now, it seems that randomly only one side of the call is connected, either we can hear the caller and they can’t hear us or we can’t hear the caller and they can hear us, sometimes when we pick up the caller is still getting the ringing tone or the “thank you for calling” message.

There are no obvious patterns in the issue, i.e. not restricted to certain numbers or regions.

It is critical that this is fixed as we are running a small call centre.

CentOS 7
Asterisk 13.4
Cisco IP Phone 7940 / 7960

In a SIP session the Callee must reponse back with a 200 (OK) after the 180 (ringing), the 200 (OK) response indicate that ,the call has been answered. Also contains a message body with the SDP media description of the type of session that callee is willing to establish with the caller .

Maybe the callee is not receiving the confirmation ACK after he send the 200 (OK)

I suggest you take a SIP trace when this problem occur also make sure NAT setting and firewall settings are properly setup and verify if this issue happen with differents SIP clients or just with your CISCO phones