I agree that it's rude, but I also have noticed that as time goes on, it seems that people just don't do this anymore.
I don't think it would be rude of you at all to call. Here's what a friend of mine wrote:
PARENTS: I need to shop for party supplies on Monday, so please be sure to call or email me and let me know if your child will be able to attend as I will only be purchasing for those I hear back from! Thanks!
My son received a party invitation about a month ago and I did RSVP

I feel better now that I did. LOL