Tapping a POTs is not as hard as you think. If I wanted to tap your phone the junction box is right on the side of your house with a screw driver, old phone or linemen's head set and cover of night I could be listening and making calls from your house. With a van, traffic cone and ladder, I'm sure I could be on pole without question. That said though I see your concern but the real concern is on a shared segment like cable. DSL is point to point to the C/O so there is less threat there. Once on the internet backbone while easy to sniff with physical access, obtaining said access is pretty hard.

Anyway SRTP is a pretty nice feature but I don't expect the VOIPO to support it anytime soon since all the partners would need to support it.