[Users] TLS / SRTP implementation

Hi, I'm a newbie in OpenSER usage and in SIP in general so perhaps it's very simple questions. I understand that TLS can be activated/disactivated from the configuration file but can not be used only according SIP number for example or call streams(internet), that's right ? What about SRTP ? In order to have SRTP working, I think I "just" need device phone/softphone with this feature. It's not dependant of the OpenSER configuration, is it ? Is it possible to force the use of SRTP when it's possible ? For this, perhaps, I need to configure "mediaproxy" or "rtpproxy" module or something else ( mediaproxy daemon for example) ? Thanks, Christophe.