It is arguably a different world today when few have local PSTN gateways and most traffic happening over the Internet.
Eavesdropping on the Internet or public WiFi (if using soft phone) is a much bigger threat than local packet sniffing.
So today, if admins don't think they need the servers' assistance with ensuring encrypted links at all times, they are insane.
There's an interesting discussion here about detection of encrypted vs non encrypted RTP: https://www.twilio.com/blog/srtp-deep-dive
Twilio for their SIP trunks has an option to enforce SRTP and TLS for SIP, so since we use them for trunks at least we are covered in that regard.
You mean a general parameter on the phones, such as setting "RTP Encryption" to ON on a snom phone?
Is there a way to check that phone settings are working correctly, except by sniffing packets and looking for, for example, "digital silence" as described in the link above?