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