It is believed that a call was made from NCs cell phone to BCs cell phone while he was in route or at Harris Teeter. What is not know is if these calls were made from the same cell tower or a different one. If they were from different towers, the phones would have to be in different locations. This would mean that either NC made the call to BC (which impacts the theory that she was already dead by 6:30 am), BC somehow set up his phone to auto answer and made a call from hers to his (although, this would be difficult if they were on separate towers due to the timing of the video of him at HT...unlikely that he could make the call, put the phone back and get to HT in that short of time), or that he had someone help him by making the call from her. They questioned MH and did the good cop / bad cop routine trying to get him to admit helping BC by making this and other calls from her phone. I'm not sure what the other calls are.
The other part to this is that her phone is locked, and according to BC, was locked when JA asked him about on the morning of the 12th. It's reasonable to assume that NC locked her phone to keep him from seeing personal information on the phone. It is not known if she actually did lock it, to what level she locked it if she did (supposedly, there is a lock level that still allows you to make calls), or if he added a lock on the 12th (or knew her password) to make it seem that the call couldn't have been done by anyone other than NC.
I tend to believe that she had it locked and he didn't know the password. It would make sense given the state of the relationship.
I agree that there are some questions and inconsistencies related to the cell phone that have not answered. BTW, I believe that HP asked BC to check Nancy's phone and that is when he said that it was locked.