Adult content blocker
I've just switched to T-Mob UK from Orange UK. My sim switches over tomorrow so I've no actual experience to go from but the cust service guy said that their adult content blocker also block IM applications so needs to be removed. You can do this through T-Mob cust service.
Hope that helps, and that I don't have the same problem from tomorrow!
White Nokia Lumia 920 from EE UK - stock rom
Matthew Ch 6 Vs 10:
"Your kingdom come,
"Your will be done,
"On Earth as it is in Heaven."