Hi shellsie,
we have a baby monitor but havent used it yet obviously. LOL! We dont have cordless phonees though so not sure on that one. I can tell you though that a gf of mine has an angelcare baby monitor and they have cordless phones and get no interference with it.

For peace of mind (when your baby is down the other end of the house) I would get one and if worst comes to worse and you do get interference and someone calls on the phone just turn the monitor off.

Good luck with the upcoming birth of your little one