pure IP: NO SMS charges, NO call/talk time charges, and NO usage charges from any company.
decentralized: I don't have a server processing all the messages (which means I can't social network your contacts, data-mine your messages, sell your info, etc).
works with any email server: use a free email account or use your own company email server if you want to keep traffic between co-workers in house.
world wide: works anywhere email works.
send when you want, listen when you want you can send anytime and your incoming messages are queued for playback when you have time (program will check the in the background and show a notification when messages are available).
less downtime: everyone is not dependent on 1 server/service so no chance of complete downtime, plus your outgoing messages are queued/sent in the background so your server's downtime will not affect you except for the speed of deliver, and finally you can use/host your own servers (many available open source and private solutions) to manage uptime/QOS.
encryption supports SSL encryption for sending and receiving of messages (default is SSL for free gmail accounts).