Set your RegistrationPeriod to 3600, enable X_KeepAliveEnable if it is not already, and reboot your device. If you use OBiTALK to provision your device, you will need to use OBi Expert Configuration to change this setting.
If that works, I would be interested to know if 900 works, and if not, then 1800.
FPL has recently started blocking customers who register too frequently and is recommending a RegistrationPeriod of 3600. I dislike registration every hour but it's up to them, not me.