The most detailed call state transition flow for a new outgoing call
would be:
UNKNOWN -> DIALING -> RINGING -> ACTIVE
But, if the modem doesn't support reporting intermediate states
(e.g. DIALING or RINGING) then a successful start() should imply
getting into ACTIVE state right away.
For now, only the Huawei plugin implements the detailed transition
support, so make them configurable via call object properties.
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