What we plan on using for Flawless eSports is the app called 'kik' - similar to Whatsapp I believe, but doesn't require a phone number. We're making a group chat, so it's easy to contact teammates when to be online and whatnot. On my phone kik only has a green light that flickers when there's an unread msg, not vibrate/sound, so it's never irritating if my teammates are talking for hours whilst im in class/work.
Something along the lines of what Jitspoe put, IRC with a script to sync offline chat for when you're on PC, and kik when you're not (via cellphone). Or maybe just kik, not entirely sure what you're looking for.
The 1 issue with kik? Sometimes will get spam msg's, but it's 1 msg from some hot girl usually, and you can choose to deny the text. Not a big deal.