We do that with a 3825; different paging system though. All users have to do is dial x1111. We use CallManager, which bounces the call to the gateway (router) since no extension 1111 is defined on the CUCM. The router is then configured to ring the call on the FXS port that we specify below, where the paging system is plugged in.
Below is what's configured on the router, with the paging system plugged into FXS port 3/3:
!
voice-port 0/3/3
description Overhead Paging 1111
!
dial-peer voice 67 pots
description Overhead Paging 11th floor
destination-pattern 1111
port 0/3/3
Also check here and here.
HTH
Ben