I moved her from the Uk a year ago. When I first came, I had a UK mobile (Three Mobile) and I got a GBP 400 bill for roaming!!!

Hala Sim card is QAR 50 (including QAR 25 credit)- Hala calls are cheap as chips - get a phone that will allow whatsapp for free text messages over wireless too!

If you bring a handset from the UK, make sure its chipped.
Hope this helps!