Hi @razorpig
Have you tried with different roaming networks and does it fail too?
And are you sending sms using international number format e.g. +91 XXXX XXXX XX or +44 7xxxx xxxxx or could it be the contact number you keying is a local number format, e.g. India: 9876543210 uk 07xxxx xxxxx, if the latter you should use the international number format.
If you tried all the above and still no joy, maybe have a chat with three support (https://www.three.co.uk/support/contact-us ).
I've had a similar problem, I was trying to text back someone in UK as didn't use whatsapp, from an european country which I cannot remember, and when I contacted support and the problem was brushed it off as I was leaving in the next day or so and contact center agent was saying that in order to troubleshoot they need me to stay for 48h or 72h.
As I type and because you are a Payg, I'm also wondering did you set any outgoing call barrings? or could be that payg costumers are barred for international calls and sms? - worth asking that to the support too.
Good luck with it.
