There are special problems with telephony, but there are efforts to address security in this place. Here is a talk from a couple of years ago : https://archive.

There are special problems with telephony, but there are efforts to address security in this place. Here is a talk from a couple of years ago : https://archive.fosdem.org/2016/schedule/event/tls_and_sip/ https://archive.fosdem.org/2016/schedule/event/tls_and_sip/ Progress does seem very slow