TS - when you say wireless phone, I take it you mean DECT, which is the standard for portable domestic telephones, although there are still some analogue ones around, which have obvious antennae.
The DECT phones should not cause a problem for your WiFi. I have one on my desk, in use most of the time, and it does no harm to the PC's wifi connection which is 30" away.
Are there any other radio devices around? Not receivers, but devices that transmit? I heard of a guy who had problems with a TV range extender, for example. It could even be that such a device next door could be causing a problem.