Heh, if my parents annoy me I tell them up-front "look, stop calling me please I'm getting tired of it, I end up talking more with you than I do with all my friends combined, i'm also tired and busy and you make it hard for me to even get a coffee or a 5 minute lap" enough said...
Why do they call you exacly? just regular conversation? support? money? I could say more if I knew.
Don't worry about being rude if it's really annoying.