Here's how my instincts tell me to respond:
"Only a fool would accept increased responsibilities and the addition of being on call without more compensation. If you honestly can't afford to increase pay at all, I'd prefer to stay in my current position and continue to do the bang-up job I've been doing. If you can afford something, let's talk."
But my instincts usually get me into trouble, so I wouldn't actually say that without taking the time to think things through.
Thinking things through a bit more, I can only come to one likely conclusion. Honestly, they already think you are a fool... and are just trying to get more work out of you. What rational person would go from 40 hours a week to 40+?? hours a week (if you don't think being on-call is working extra hours, you've never done it) plus new jobs like training other people, and no additional pay?
I'd be scared if I were you. Either they are entirely stupid (possible), or they just flat out don't need you at all and consider you expendable/replaceable with the next coder that walks down the street, or they would never even ask something ridiculous. Or you've demonstrated them in the past that you were willing to accept ridiculous things, I dunno.
Are you sure they need you? If so, you have some room to be polite and not burn any bridges but stand firm that more responsibility will only be accepted with more compensation. If they don't need you, they are playing you and you should keep your mouth shut while looking for a new place to work. (and not accept the promo in as few words as possible)
Personally, I'd probably confront them about the absurdity of their proposal, but I tend to be direct like that, everyone knows where I stand when I work with them and I'm not afraid to say unpopular things. It's gotten me into trouble plenty of times; I don't necessarily recommend it.
Do what YOU think you should do, not what some stranger on the internet says. (Though sometimes I agree it's useful to hear a variety of opinions to see if you've missed any angles)
EDIT: Okay maybe I wouldn't confront them. At least not for a few days, I'd let myself cool for a while and think about it. Please don't read my post and go confront them and get fired, if you need the job that is. But don't voluntarily agree to make yourself miserable with more work for no additional benefit.