Originally Posted by
LarryJ
That was true but not anymore.
The 737 fleet was entirely on the Thales system which is the system that (unofficially) supported iMessage.
When the 737-9 MAX aircraft arrived, they introduced a new system, the ViaSat system. ViaSat has an expanded coverage area and better reliability but does not seem to allow iMessage without purchasing internet access. A number of 737-900, A319, and 757-200 have had the ViaSat system installed as well.
These are the same system, just slightly different generations of hardware. LiveTV (now owned by Thales) provides the aircraft side hardware, and ViaSat operates the satellite. The expanded coverage is due to support for ViaSat-2, instead of just ViaSat-1.