Call connected tests

[sparre]

We want to be able to identify if a call is connected (from one basic_agent to another).

Ideally, we should also be able to recognise if a basic_agent is receiving a dial-tone.

[rostgaard]

Early media detection is part of the PJSUA API. From basic_agent.c we can detect what state the call is in by tracking call state via the on_media_state callback. Polling the call list via the process pipe would probably be preferred rather than blocking-read for update changes.
The call list would also hold the information about the caller_id of the callee.

To detect if the agents are truly connected we could:

a) Send a sine wave[1] and detect volume level
b) Send agent id as DTMF tones[2](see note [3])

[1] http://www.pjsip.org/pjmedia/docs/html/page_pjmedia_samples_playsine_c.htm
[2] http://www.pjsip.org/pjsip/docs/html/group__PJSUA__LIB__CALL.htm#ga902a120fe569bee0b49dac995a09eb14
[3] http://trac.pjsip.org/repos/wiki/FAQ#dtmf

[rostgaard]

Have we reached a decision on this one?
As I recall it, we are using media_state (or similar) callbacks to determine whether or not there is an incoming call.
Is this sufficient?