Nobody has a bloody clue
My daughter has recently gone to Melbourne for a year of study and took her o2 phone with her as they do an 'unlimited roaming' thing where it costs £5 a day for as much data as you like, but apparently it's throttled at some indeterminate point. The plan is that she uses that for the first few days and then gets an australian sim card.
Anyway she'd been happily using this until suddenly she found that whist perfectly able to browse web pages without any slowness or any problem at all, all her messenger apps: facetime, iMessage, facebook messenger and whatsapp just plain refused to work at all. She was able to send and receive SMS. So I started a chat with o2 and they went through the obligatory 'take your sim our, wipe it with a soft cloth and put it back', and then abruptly ended the chat.
So I tweeted at them, and I had someone reply who initially seemed quite helpful. They asked pertinent questions and then came up with this absolute fucking gem of a response:
“Ah okay, we're unable to guarantee speed and strength of signal whilst abroad as you're roaming on another network. As your daughter can access the internet through a web browser, it shows that her phone is able to connect, but that the connection isn't strong enough to be able to send messages by iMessage and Whatsapp. We're sorry for any inconvenience this may cause.”.
It started working the next day anyway.