"access point" settings pops up after each restart

[Homura]

Hi, 

I am using Nokia Lumia 520.

Every time I start the phone, after it boots up the window "access point" appears.

Just recently I was using other Lumia 520 which I bought sim free and I had no such problem.

This phone I am using at the moment I received from O2.

Looks like I am the only one experiencing this issue?

Phone reset did not help.

Regards

[perksie]

It reads that it won't work if the phone is locked to the network.