I reckon it's the phone, but I figured I'd ask before taking it back. I've just bought a BT Studio 4100. It's to replace the binatone digital phone which is a bit old and the batteries in noticeable decline. It's not a BT line, it's through TalkTalk. The binatone still works through the socket as does the internet (using the same connection to write this) The 4100 is fully charged, plugged in etc. There's no dial tone Swap the Binatone back in, there is a dial tone Dial in from the mobile, with the 4100 connected, the line rings - the 4100 doesn't dial in from the mobile with the binatone, the binatone rings.