Just to clarify, there is no option to send an IM message to a phone number.

All of the IM networks available expect the messages to be sent to another IM user on their network.

The only way you can get a message to a phone via IM is if that phone is also on IM. Or convievably there could be a IM->SMS gateway that converted your IM message to SMS for example (perhaps via Google Voice).

So yes, you will need to join an IM network to send IM messages, and you would need those people that you want to send message on to also be on that network. You would then add them as contacts (using their MSN or Gtalk name for example) and then you can send them IMs.

I suspect that you have somewhere gotten the impression that IM is a method for sending messages to phones - it isn't, not in the way that you think.