In WoD the guild switched to Mumble hosted by TypeFrag and enjoyed its features. We also migrated our text chat From GroupMe to #Slack. These both served us well however a new contender has entered the market place.
Discord - https://discordapp.com/
It is in early release at this time and offers the easiest registration options of any of the products we have used before. In addition to this it couples the benefits of Slack & Mumble in one with the price tag of FREE.
That right high quality easy to use VOIP service with the awesome mobile chat experience - both of which are available cross platform.
Log into any of the guilds Clan presences or in slack chat to get the invite link.