To topup an T-mobile uk sim card abroad try.
#33*1234# SEND
it will do something (requesting action)
then say barring cancelled.
Wait for a callback phone call from T-mobile
voila the topup line.
the voice will ask u enter top up code or talk to customer services.
works for me in china
Vodafone UK (90 day expiry) so be careful, T-mobile and others are 180 days.
Virgin Mobile is 1 year but wont roam without credit agreement signed.
O2 generally does not roam that well.
Vodafone is great worked everywhere i went.
T-mobile 2nd great. I can talk to customer services for free from in china.
costs them