My iPhone works fine via 4G and WiFi if I make calls to our internal extensions, but when I try to dial an external number, for example my friend's number, or my house, when I type the number and press on dial, it hesitates for a millisecond and it takes me back to dial the number. If I dial an extension it works fine. It does this from the WiFi AND the 4G connection. The phone is On Hook (green) and Available. Any help will be appreciated. Thanks!