The outcalling number must start with 9*. Voice mail ports always access Intercom first, so 9 gets an outside line, * pauses for 1.5 seconds before dialing, to allow for the dial tone to connect.
Pagers are finicky. The paging service ususally uses inward DID trunks, and the number of rings before answering can sometimes vary. You must dial the pager number, wait for the service to answer, and then input the number to be displayed. You will put in something like 9*5551212*****12#, to get dial tone, wait, call the pager, wait, put in 12 to be displayed.
Play with the pauses between the pager number and the display number. Not enough pauses will cause it to put in the display number before the service is ready. Too many pauses will make the service time out before the number is input.
I personally use 9*5551212*12*12*12*12*12*12*12*12#
This makes me get a display of anything from 12 to 1212121212121212, but anything tells me to call in and check for messages.
Finally, do you have a butt set? Monitor the line as it outcalls, so you can hear the timing, and adjust as needed.