I need some help with replacing some single line telephones. The school has a Bogen 2000 intercom system and is about 10 years old. I have found out the old Bogen has a 25 volt ring voltage. The existing sets are NEC DTP-1-2, the orginal installer removed the speaker and installed a PKM11-4A0 buzzer. Does anybody know of any other type or brand of handsets that will work? Thanks

Those phones are sold by Bogen under the part number MCESS and MCWESS. It's a standard telephone with the ringer removed and a piezo buzzer installed in it's place just as you have mentioned. You could probably make them yourself with a little bit of effort.

https://www.bogen-es.com/multicom/documentation/docs/MCESSs.pdf