I'd suggest a DECT phone (digital cordless) but I'm not sure if they (or any kind of digital phone) are available in the US.

Over here you can get one for less than 50UKP.

Edit: Discovered that there are digital phones in the US, but there isn't any real standard. Saw mention of something called MARS, which seemed popular though.

Is the wireless unit you use for your TiVo digital or analogue?