Version Core 4.4.0-13-gc99cb9c88 Appimage
The server/proxy is opensips. The certificate that is installed in opensips
works for other user agents. Linphone rejects the certificate. The certificate
was generated by Lets Encrypt.
2020-11-08 15:23:44:071 [AppRun.wrapped/belle-sip] MESSAGE Channel
[0x4c70290]: SSL handshake in progress...
2020-11-08 15:23:44:091 [AppRun.wrapped/belle-sip] MESSAGE Found certificate
depth=[0], flags=[not-trusted ]:
cert. version : 3
serial number : 03:3D:58:6A:10:1B:E4:D8:68:7C:2F:14:41:57:D4:C9:D0:8B
issuer name : C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
subject name : CN=bogus.com
issued on : 2020-09-25 15:29:57
expires on : 2020-12-24 15:29:57
signed using : RSA with SHA-256
RSA key size : 2048 bits
basic constraints : CA=false
subject alt name : bogus.com
key usage : Digital Signature, Key Encipherment
ext key usage : TLS Web Server Authentication, TLS Web Client
Authentication
2020-11-08 15:23:44:091 [AppRun.wrapped/belle-sip] ERROR Channel [0x4c70290]:
SSL handshake failed : X509 - Certificate verification failed, e.g. CRL, CA or
signature check failed
That sounds symptomatic of Linphone either using its own CA bundle, which may be out of date and doesn't include the Let's Encrypt Root CA certs, or the app is not able to query the system CA root bundle to validate your end entity cert.