Merge pull request #21 from martinjones/master

[voicecall] Various fixes and improvements.
Merge pull request #22 from martinjones/master

[voicecall] Provide a valid startAt time.
Fix warnings and duplication in voicecall code
Merge pull request #23 from jpetrell/master

Fix warnings and duplication in voicecall code
[voicecall] Improvements to ofono plugin.