Looks a common problem. See these:
https://community.cisco.com/t5/ip-telephony-and-phones/phone-cuts-out-15-minutes/td-p/4623512
https://btbusiness.custhelp.com/app/answers/detail/a_id/41858/~/im-having-problems-with-sip-and-calls-are-cutting-off-after-15-minutes
https://community.cisco.com/t5/atas-gateways-and-accessories/spa-122-hangs-up-after-600-seconds/td-p/2203108
Ekrifj
November 2022