Hello @edisondotme,
It could be a Linphone issue, or a SRTP issue (usually configured at the same time as your encrypted SIP traffic).
I do not remember having a great experience with Linphone. You might want to try others too. I shared a bit of my experience in this regard here:
Check to make sure SRTP is configured properly, then if it still does not work, give a quick try to other soft phones. Like the one from voip.ms, or Zoiper. For my part, I paid for Groundwire and it is just working well, period.
Also, check if on your router you have disabled the “SIP ALG / SIP Passthrough” option. Very often, it causes problems with SRTP.
Kind regards!