Wireless phones are made from extremely CHEAP components and it's all together possible that a breakdown of some component in the phone base station is causing RFI to interfere with your WiFi.
Since you probably don't have the sensitive instrumentation to detect RFI, about all you can do is troubleshoot by replacement.
I'd go down to a local store that has a money back guarantee on their products and buy me a new phone.
Then try it out and if it doesn't solve the problem, take it back for a full refund.
You could do the same for the WIFI device on the ???
You didn't even mention what your WiFi device is..... a desktop? a Laptop? a computer game?
Please fill in all the blanks so we won't have to just guess about what we're doing. Eh?
I have a wireless G adapter in my Acer Netbook, but I've disabled that to use a Wireless N, USB device, with a 7db gain antenna on it.
It's much more reliable over distance, than the Wireless G device in the PC.
Good Luck,