We've used a couple different products. There's an Algo SIP Pager/Ringer that we've used through a teleworker and locally. We've had problems getting the paging side of it to work through the Teleworker, but that was secondary function.
We've also used SIP ATA's with Valcom PA-2A, that will do loud ringer.
In both cases, the ringing was done through personal ring groups.