base-call: Increase incoming call timeout to ten seconds

The standard says a RING or CRING should be emitted whenever the network sends an incoming call notification, which is nation-specific:

"Interpretation of indications from the network to determine what constitutes a "ring" is defined by national regulations. This result code should be repeated each time the network repeats the incoming call indication." -- ITU-T Rec. V.250, p. 42

On the giffgaff (O2) network in the UK, a SIMCom SIM7100E modem emits RING indicators every six seconds. The current timeout is only five seconds resulting in a stream of timed-out ModemManager call objects whenever there is an incoming call. To fix this, we increase the timeout to ten seconds.