I unlocked a Netherlands T-Mobile T230 yesterday and all seemed fine.
My customer was on Vodafone Ireland.
Today she contacted me to tell me the phone was making and receiving calls ok, but she could not send SMS messages although she could receive them.
I went through the settings on the phone and message centre number etc was all correct, so I did a full reset on the phone and now it is working 100%.
I just wanted to post this for others who might have the same problem and think that the only option is to flash the phone. Hope it's of use to somebody.