It depends on what model phones you have and what type of paging interface.
You can do it with a 5485 IP Page Adapter and pretty much any model phones, put the 5485 in a page group with the phones and connect the output to your paging system.
Or, if you have 6900 series phones and are running MiVB 9.1 or newer, you can use an Algo 8301 as your paging interface and a multicast paging server and do multicast paging to the 6900 series phones.