We have two clusters -cluster with MGCP gateway PRI is 18.104.22.168900-1 (Cluster A) and our cluster handling our contact center is 22.214.171.12400-5 (Cluster B). 1 in 4 calls going outbound from Cluster B through non-gatekeeper ICT to Cluster A disconnects with a fast busy immediately and exactly at after 10 minutes and 20 seconds. This is a consistent pattern. This only happens with outbound calls through the PRI. Inbound and outbound calls between the cluster is unaffected. When I do a packet capture off the phone, I can see that as soon as the my would disconnect with a fast busy (7961), there is still an RTP connection with the mgcp gateway but only 1 channel. The PSTN phone on the other end is apparently still retaining the channel and finally drops off after 5 minutes.
This is what I've done and the results are still the same:
a. Made both ICT trunks use MTP.
b. Changed route pattern call classification as OnNet and OffNet
c. Upgraded firmware on the phone.
Situation still remains the same.