as of April 2019. Some like Lancom call this feature ‘Voice over Secure IP’ (VoSIP). Some like innovaphone call it ‘SIP Secure’ (SIPS).
The idea was to test sRTP (with SIP-over-TLS at 192 bit) over IPv6 (with DSCP EF). Additionally for SIP, Session Timers, Compact Form, SHA-2 Digest, and DNS-NAPTR were enabled on the server. Below is the configuration of each phone. That should enable you to add that phone to your own VoIP/SIP provider like Digium Asterisk, DUStel, or Easybell Germany. In March 2019, Easybell added the server secure.sip.easybell.de which encrypts always. That was not tested by me, yet. Instead, I used the optional encryption server, which is still available at sip.easybell.de. IPv6 got its own sub-page …
Many of the softphones, like Bria and Linphone, are not only available for Apple iOS or Google Android but also for computer systems like Debian/Ubuntu and Microsoft Windows.
Some network operators call those Customer-Premises Equipment (CPE) which is the broader term for such DSL modems.
All manufacturers above faced at least one security issue. Many of them had fixed the issue(s) when publicly presented in March 2019. Manufacturers in red failed in April 2020 = twelve months later, to fix their remaining security issues. Manufacturers in blue failed in October 2019 = six months later, to fix their remaining security issues. The following manufacturers are/seem to be out of business: CEM Solutions (brand: ALLO CIP-100), Moimstone (brand Apivio), and Tecom (brand Mocet). Technicolor (former Thomson) used sRTP in the past but I am not aware of any current implementation, which offers sRTP. If you know more VoIP/SIP manufacturers with sRTP for voice and HTTPs for their Web interface, please, notify me!