If that happened -- the cell phone records would be simple to obtain, of course -- it's a slam dunk mistrial and a likely prosecution for the contemptuous juror.   

Allowing the week-long "vacation" during deliberations was also a boneheaded move.  Way too high a risk of jurors being swayed improperly by outside influences.  Actually, I'm kinda surprised they weren't sequestered for the duration of deliberations since this was a high-profile, high-emotion trial in the same jurisdiction where the murder occurred.