I bought some philips cordless phones with the answering machine a while back. I think they're nice. 1.9Ghz so they dont bother wifi laptops was my initial main reason for buying.

They use regular old standard AAA NiMH rechargable batteries which you can buy replacements in most any drugstore for resonable money.. when they do poop out someday.
Standby time : 120 hours
Talk time : 10 hours

Only negative I see is they are tiny - but do work nifty for me, even on hands free speakerphone too.

Here is a link that might be useful: Philips DECT Phone/Answering machine

I recently got a VTECH cordless with answering machine. Mine also came with an optional second handset that only needed an electrical outlet, not a phone outlet.
Long life batteries, good range for wandering around, adjustable volume for when I'm in the shop.
It's 5.8GHZ. I'm not using WiFi, so I don't know if there's an interference issue.