Go to the manufacturers website (for the phone) and see if they have a product data sheet. Should list the operating channels and spectrum. In some cases these will cover basically your entire router spectrum, but you may find some gaps. I have always foufd that you need to test on channel 1, 6 and 11 to get an idea of where the phone is. But by keeping to the extremes you run a better chance of finding the best channel.
That or get a new phone.