Open up iChat, create a new chat (cmd+n) and add a participant person (click the + and choose "other"). In the buddy name input box that pops up enter: +1(area code)(telephone number) and click ok, then type a message and send it.
Open up iChat, create a new chat (cmd+n) and add a participant person (click the + and choose "other"). In the buddy name input box that pops up enter: +1(area code)(telephone number) and click ok, then type a message and send it.