Many things can interfere with a wireless connection. A cordless phone, a microwave oven, a baby monitor.
You've identified it as the phone itself, solution would be to change physical phone to either a corded phone or a cordless that is NOT 2.4GHz range. leaving you with 900 MHz older tech or 5.8 Ghz newer tech. NEW 5.8 GHz cordless phones: are they better than 2.4 GHz?
is a good explanation of the frequencies so you can decide. As they state in the article, if you're buying it to deal with interference make sure you get a full 5.8GHz phone not a dual-mode that can give you better battery life but will still use the 2.4GHz and interfere potentially.