BBM.
Exactly. That half hour gap could have been the bin lorry rumbling along a largely empty road at around 12mph OR a car with Corrie and his phone in pulling over for a period of time for some reason or other before going on.
If they were heading for a party and had already been drinking, maybe Corrie or the driver felt really sick and stopped to throw up.
Or, IF this was Corrie being assisted to go AWOL, time for the car to pull over for quarter of an hour or so and the driver ask, "Are you sure you want to do this, mate? There's still time to change your mind." Or time for Corrie to change clothes into something less noticeable.
As I've previously said, I do not think the phone was in the bin lorry. So I'm really looking for a scenario in which it would take the phone an apparent half hour to cover 6 miles. As we've both said, a car stopping for a while in the middle of nowhere between the mast catchment areas would provide that scenario.