My wife and I have the same phones and the same services on a single account.
When I set the default voicemail greeting on my phone, it says "<phone number> is not available. To leave a message...". When I do the same on my wife's phone, it says "<phone number> is not available." but the next part is not there and after a couple seconds it beeps to start recording. Adding in our name to replace the phone number has the same difference in results.
Is there something that may be different in the phone settings I should look for?