Found this siude effect with a user whose phone and other ios devices
stopped syncing

He using Gw 2012 SP4 with GMS 2.1

We tried all sort of things to get his phone to re-sync (resync phone,
re-initialize user, gwcheck, rebuilt the phone from scratch, update the
ios, etc).

Interestingly, on the phone itself (we tried a couple of phones), the
phone gets very hot and the battery is depleted very quicky if not
plugged in.

In the end I found that he had dragged an Address book into his mailbox
in GW.
Once that was put back in its proper place all was well.