My Samsung Galaxy S2 keeps saying not registered on network when I try to phone
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.