Well after youve offering your number to her she will most likely give you hers. Say after shes gave you hers something like "hey if youd rather Intant message we could do that instead, do you have an IM account?" then she would tell you if she does or if she prefers the phone over IM. usually if she has no problem giving out her number she wouldnt mind giving you her IM. if she doesnt give you her number after you offer yours then dont ask for an IM