Any normal/softphone works perfectly, but Sangoma (Digium) phones uses an SSL certificate to encrypt phone configuration messages. That certificate expired on May 28th, 2021. The certificate is embedded in the DPMA blob, so it can't be renewed.
Basically this is the cost of putting binary blobs on your server :(
The solution is to download a new binary blob from digium and to replace the existing one in /usr/lib/asterisk/modules
check https://my.digium.com/en/docs/DPMA/dpma-download for the right DPMA module.
# cd /root
# wget http://downloads.digium.com/pub/telephony/res_digium_phone/asterisk-13.0/x86-32/
# tar res_digium_phone-13.0_current-x86_32.tar.gz
# cp /root/res_digium_phone-13.0_current-x86_32/res_digium_phone.so /usr/lib/asterisk/modules
# asterisk -rx "module reload res_digium_phone.so"