4 years ago I flew from LHR via BKK to SYD.
maynotbe perfect,
I had the opposite experience. LAX to BKK. Welcoming messages from mobile companies in Alaska, Russia, China and Taiwan.
This happens due to the aircraft being in a high-angle antenna sidelobe long enough to make a connection. I expect you might be able to send an SMS during one of these brief connections, but I doubt you could make a voice call.