You have outcalling turned on and misconfigured for mailbox 9991. And the outcalling number must be starting with 70, or 8 and the paging line port, which is the external paging access code.
(Normally an outcalling number starts with 9*, and then the telephone number. The 9* gets an outside line and pauses for dial tone, then dials the rest of the number. In your situation, it is not accessing an outside line, but rather dialing on Intercom, and the beginning of the number matches the paging code)