not..........the BEST way is a two step process from any ONLINE forum, included networking sites like Myspace.
U did step one........chatting with her..........not sure if you are exchanging emails or actual IM's.
Extrapolating a # from an email is tricky right off the bat........unless you been exchanging emails for a bit.
U want to transition into a IM conversation...........so even though she appears OFFLINE........she may just be hiding, to avoid getting IM by everyone. So hit her up when she appears offline.........leave a lil note and ask when she will be on.
U can use the email..........but you gotta be enticing enough for her to warrant giving out hre number..........just make sure to ask whens a good time to call.............THEN DONT CALL HER THAT EXACT TIME.
ANYWAYS.........the 2 step process is: ask in email what her IM name is.......then converse in the IM forum............THATS WHEN U say "HEY whats your number.........I gotta hear your voice" OR " lets see if you can keep this up on the phone or I bet you're actually a dude..........whats your number so I can verify for myself".
Texting is best just prior to calling so a chic knows its you...........