Why is my phone defaulting to Fizz EXT?

You're not the only one who has had spotty text messages over the last week so I don't know if that's really part of your Fizz Ext problem. I don't know what kind of phone you have, but if it's an iPhone maybe there's a system update you can download over WiFi that will help. If it's an android, maybe you can change your preferred network type to change what you connect to? In either case, if you haven't already tried rebooting that would be a good idea. You can also pop the sim card out, reboot, and put it back in. Sometimes that gives a phone the kick in the butt that it needs.