My Samsung Galaxy S2 keeps saying not registered on network when I try to phone My Samsung Galaxy  S2 keeps saying not registered on network when I try to phone also a no entry sign where the signal bars should be keeps randomly appearing.

Does anyone know how to fix it or whats happening?
It sounds like you may need to reset your APN settings, Your phone will be slightly different than mine, I'm using a Nexus 4.  Try locating the following:

Settings -> Wireless &amp; Networks -> Mobile Networks -> Access Point Names -> and from there locate the "Reset to default" option.

If that doesn't work, you can backup your device, and factory reset it.