help, my phone thinks it's on O2 when it isn't!

So I was at a Brixton Academy last night and they were doing 2 for 1 beers if you were an O2 customer. Being a tightarse I whatsapped a pal and used their number and password to sign into the O2 app. Only problem is shortly afterwards my phone went to emergency calls only and I seem to have locked it to O2. It's a blackberry from carphone warehouse and so is (was) unlocked and I'm with EE. Any ideas how to get back to recognising the EE sim please?