Stop YIM from automatically going to my cell phone when i sign out on computer?

I checked off a box for my messages to go to my cell when i signed out only because i thought i could change that option whenever i wanted to. but it goes directly to my cell whenever i sign out on my computer and i want to change it back so it doesnt do that. i have a pay as u go plan and every time i get ims there it uses up minutes. is there a way to switch it back? i'd like to be able to switch back and forth at will if that is possible. does anyone have any suggestions???