
Tell them they've already called you today, sound annoyed.
Ask them to explain all about their product, put the phone on the side and walk away.
Say 'we already have one of those' and start complaining about it.
Start trying to sell them something.
Tell them that you're actually a slave and unable to purchase anything without your masters permission.
Explain that you're halfway through a vital voodoo sacrifice, can they call back?
Ask them if they're a scientologist, as you only purchase from authorised Hubbard retailers.
Gasp down the phone and explain how you dreamt about this only last night, go into details.
Muffle the phone, apologise and say it's because it's hard to talk in your gimp outfit.
Ask them if they are 'the one', calling to free them from the matrix.
Congratulate them, and tell them they could be a winner if they just dial this number...
Pretend to be an answering machine, really badly. "No I am an answering machine, please leave a message. Beep."
Apologise and say that your current persona does not take phone calls, can they call back when your alter ego is in?
Buzz your drill near the phone, explain you're testing powertool cartilage damage.
Ask them if it comes in puce/tomato flavour/B flat (whichever is least appropriate).
Tell them that "My granddad had one of those and it caused him a nervous breakdown."
Say that if they wish to talk to you it'll cost them £2.99 a minute, breathe heavily.
Tell them you're about to return to your home planet and can't take the call.
Shout backwards after badly muffling the phone "No it's *company* - shall I ask if they want to sell their soul?"
Ask them to explain, put on a record, start singing along to it.
Start taking their order for pizza.
Ask them if they're product is humane/vegeterian/vegan/free from human contact/produced in a gravity free environment/quark free... (etc)
Ask if they can send you details? Ask for their name. Ask for their mailing address. Give them their mailing list and the callers name as details.
Tell them you have an embalming appointment and can't be late.
Speak perfectly calmly and reasonably, fire a cap gun, scream, drop the phone and speak in a different accent saying "I told you never to answer the phone."
I can't think of any more... suggestions welcome :)