The UTI-1 or Universal Telephone Interface sits between the phone system and the paging system. Then, any phone in the pbx should allow a preset number to be pushed to begin the page. Some phone systems don't need a UTI-1 for paging. Take Allworx for example. The Music-On-Hold (MOH) port also provides paging. In this case you can just use a Y-Cable to wire both the MOH and paging from the same output. When in doubt buy it. It can always be returned in 30 days if it's not needed.
Another option you have when using the UTI-1 is self-amplified speakers. Feel free to drop the amp all together and wire your speakers directly to the interface. I personally don't recommend this option because it limits the kinds of Bogen speakers you can use.
The UTI-1 is $158.81 and can be wall mounted or rack mounted using the RPKUTI1 which should only take up 1AU.
For more information regarding Allworx paging systems call 800-335-0229