Let me preface that I have not been to BGT since before Gwazi opened. While the above is true, it's going to depend on how clearly the no cell phone policy is posted and audibly communicated.
Given the number of cell phone incidents on RMC coasters which have resulted in metal detectors, locker usage enforcement, signage, and audible recordings, etc. If these aren't in use at BGT it can open up a small window of liability. Again, given the number of incidents with previous RMC coasters, they can't hide behind it's the person's fault for being their phone on with them. Share the blame, absolutely.