Swapped SIM in One S - Emergency Calls Only

[Jay3GSM]

I've swapped the SIM in my HTC One S, to run it on a different number. The last time I did this, I got a message on the screen saying "Emergency calls only". I remember there is a simple setting that needs to be enabled or switched or something, it is nothing major, I just can't remember what it is!

Can anyone shed any light on it?

Thanks