Improved user feedback when a call is abandoned or dropped

[DingoOz]

Goal:
When a call has been dropped by a party or the connection between the callers has failed additional audible and visual feedback is required to inform both parties of the current status.

Tasks:

• audio loop while the other party is ringing.
• audible pickup & call end
• audio loop after no received audio for $TIMEOUT

Relevant Code References:
https://github.com/servalproject/batphone/blob/development/src/org/servalproject/batphone/CallHandler.java#L412

https://github.com/servalproject/batphone/blob/development/src/org/servalproject/audio/JitterStream.java#L245

Related Goals:
Whilst not part of this issue, future work on the following may be related:

• indicate call status is on hold,
• facilitate and identify the use of a speaker phone,
• call waiting,
• conference calls.

Background information:
Current build includes the following characteristics:

"We currently give up on a call if;
the other phone takes more than 15 seconds to "ring"
the other phone takes more than 30 seconds to answer
no network packets arrive for 30 seconds."

Source: Jeremy Lakeman 13.45 on 25 Feb 2016 - Google Groups Serval Developers.

Additional information:
Google Groups Serval Developers thread

[DingoOz]

I am making progress with this ticket and feel I'll have a pull request in the coming days.

[DingoOz]

I continue to make good progress on this.