Various W56H Cannot Dial out "forbidden"

[CloudNet Group]

We have many W56B/H in use. We are continually receiving complaints that the every month or two, a handset can receive calls but cannot MAKE calls. Display shows "FORBIDDEN via IP line" unless the Handset is rebooted. In locations with more than one W56H in use, one handset will still be able to make calls but the other will not. Incoming calls are fine, we have valid registration to the base. What will stop the handset from being able to make calls when the base is properly configured?