The call volume driver listens the speaker volume and microphone
gain from the AG. It also allow to synchronze the speaker and
microphone volume changes with the AG through AT+VGS and AT+VGM.
Volume level synchronization from HFP spec 1.5 Section 4.28.2
is performed on startup.
This driver handles phone informations about registration status, signal
strength and roaming status listening +CIEV commands. It also gets the
Network Operator name with +COPS commands.
HFP voicecall driver uses AT+CIEV indicator to notify call status
update according to Bluetooth HFP spec v1.5. This patch only adds single
call support.