Kamailio TLS connection lifetime

 TL;DR;

Pay attention to 

modparam("tls", "connection_timeout", 600) # Default

 A bit longer.

Usually, you don't need to use SIP OPTIONS ping over TCP connections. Because you have TCP keepalives built-in. And the default registration time is 1h, which is also generous in the case of TCP (TLS). But the default connection timeout, found in mod_tls parameters is only 10 minutes. So, if your registration time is lower than 10 minutes and there are no calls on this phone (which is ok as well), you'll have a TLS connection drop every 10 minutes with a corresponding indication on the phone itself.

 Again, defaults might not be 100% sane.