Just to make it even more unhelpful there are two numbers to consider, length and start time. By standard phone billing a length would be rounded up and according to reports a T-mobile start time is rounding down. So the call probably ended at 7:16 plus or minus 59 seconds with both extremes unlikely. (ABC had a photo of the T-mobile log showing the 7:12 start time but I've never seen evidence confirming the ~4 minute call length.)Rounding the time down seems rather unlikely, so I suppose there might be up to 59 seconds between the end of the call and altercation. That really doesn't say much then.
Regardless of rounding according to TM's girlfriend the call end time is exactly when the fight started when (she believes) GZ pushed TM and his headset came off.
(There may be an issue with the T-mobile clock being out of sync with the 911 clock, if there isn't the GF's version of what happened seems very problematic for her credibility. GZ told the 911 dispatcher that TM ran at 7:11:40 and it looks like the girlfriends account covers that event but if she called at 7:12 (up 7:12:59) then she didn't actually have the conversation she reported.)