We'll probably never know the exact content of the conversation. This doesn't mean that the existence of the call isn't a crucial piece of evidence. As I posted earlier it effectively counters GBC's alibi of being in a deep sleep all night.
This isn't a good look to a jury and the defence team will be hard pressed to counter the prosecution argument that he has lied to police.
The only way that the defence can explain this discrepancy is to introduce evidence of their own, namely sworn evidence from GBC or the person on the other end of the phone. The problem with this strategy is that it then opens them up to cross examination and I wouldn't be advising either of them to step into the witness box.
Even if the content of the call is never determined, the mere existence of the call is a major problem for the defence.
MOO
Sleep texting happens.... just sayin'