Well, to make it simple: When you lose a cell phone, you automatically call them and terminate the service. This could have been the accuser or her pimp/boyfriend. When the guys picked up the phone in the yard the next day, they tried it and it didn't work. That's how they knew.
My contention is that if one is brutally gang-raped, kicked, beaten and strangled - CANCELLING your cell phone service is not a priority when you're still recovering from physical injuries.
In fact, to be consistent with the victim of a heinous crime with a stolen phone - supposedly by the same perpetrators - the phone would be left in service in order to bring the perps to Justice.
The very day she is released, she can't make it to the police and can't provide them any details, but she can remember to cancel her phone service.
I think the phone service was cancelled so that others in the AV's inner circle would not be identified by the authorities. I'm sure they were not thinking Defense at that point - but the authorities. Kim had a warrant out for her arrest, did the AV's boyfriend also? Was this overriding impetus for canceling the phone service?