Ask HN: Best Messaging Platform with Presence Awareness?

My team is scattered throughout two offices and several telecommuters. I've been trying to figure out what the "best" chat platform to use for us would be. I'm not sure how others handle real-time communication, but in ours lag time of not knowing if someone is at their desk to answer a question causes a few problems for us.

These seem to be the points that no one platform seems to have:

* Contact List - An at-a-glance list of all everyone instead of the new Hangouts/iMessage format

* Active/Idle/Away - Presence awareness based on computer activity (webapps like Slack rely on window activity) so we know when someone is at their desk as opposed to off-site

* Idle/Away doesn't break when people have the mobile version installed on their smartphones. Skype is the biggest offender in this category.

* Group Chat - It should work for everyone

* Cross platform - We have Macs, Windows, and Linux users. Smartphone app would also be great.

What are others using, does anything out there get close to what I'm asking for? Or am I tilting windmills?