I hate BRU too - so much that I refuse to shop there anymore....Buy Buy Baby's return policy is so liberal - if they carry it, they don't require a receipt. So they get all my business, even though they can be slightly more expensive.
I know that you're looking for help with a new monitor, but I can't stand the new BRU return policy, and I don't think you should lose your money! So the big question is - do you still have your receipt, or is that why they wouldn't return the monitor?
I have an expensive monitor - one of the video ones...I had issues with it a few months into using it, though I don't think that it's related to the cordless phones (I also have three in my house, and they don't interfere with anything else). I called the manufacturer and they replaced it. So you could try that route.
Second, if you DO have the receipt (and have the fight in you!), call the BRU offices. You need energy to deal with them - I had gotten something off my registry that they didn't carry in the stores, and they wouldn't take it back in the store, and the website only wanted to return it and credit the gift giver, and not give me credit. I was still experiencing PP blues, and all I could do was cry. It's been too long for me to have a leg to stand on, but I would totally fight them now....
OK - I don't know if I've helped at all. I hope I didn't make you hate BRU even more! Good luck!
(AND, let me know if you get a resolution with the monitor you already have...I love to know what works with these companies and what doesn't!)