I have experienced the following issue with Zoiper specifically (I have a support ticket open with them currently) and was wondering if it's something that could/should be fixed FreeSWITCH side.

If rtp_secure_media is set to optional, and SRTP is disabled on Zoiper, when FreeSWITCH sends the Re-INVITE, it sends crypto lines. Zoiper then replies with a 514 Unsupported Media Type and the timer refresh fails, causing Zoiper to hang up the call.

My current feeling is that as Zoiper ignored the crypto on the initial INVITE it should be them that then handles the Re-INVITE consistently by also ignoring the crypto lines but I'm not familiar enough with the RFCs to make a judgement.

The workaround is to either enable SRTP in Zoiper or don't set rtp_secure_media to optional when Zoiper is in use.