Nokia 6230i Dual Sim

[fly258]

I have just fitted an advanced dual sim card adaptor to the above phone. A Vodaphone contract sim and an O2 PAYG. It works most of the time but occasionaly when trying to change networks I get "phone restricted" and it asks "enter restriction code". Where do I get the restriction code from and why does it ask for this code only sometimes? The only way I can work around it, is to switch it off then on again.