Let's start a repeater controller project. Right now my thought is to make it PIC-based. We have already built the PIC of an ID'er created by Bob Anding. We played with some of its parameters and code. Started another version of the code from scratch, written for the PIC 16F690 (Documentation).
We currently have two peripheral chips to review and consider. One is the MC145436 (documentation) touch tone decoder chip and one is the ISD1000 voice recorder chip. Both these chips are somewhat old. It looks like the MC145436 can be replaced (not pin-compatible) with an MT8870. The ISD1000 has many newer versions, some of which will connect to a PIC via an SPI interface, like the ISD1760 (datasheet, design guide, family information).
I also found a PIC-based repeater controller design which uses the MC145436 and may be worth reviewing (VK5DJ Repeater Controller).