T-Mobile USA rolled out T-Mobile @Home and Unlimited Hotspot Calling plans that let you use specially enabled T-Mobile cell phones to make unlimited calls over WiFi for a flat $9.99 per month a while back. So, their news business focused plan seems like the next logical step... (from Reuters)...

T-Mobile USA kicks off corporate Wi-Fi push

This plan is limited to BlackBerry phones. But, given the BlackBerry's popularity in corporate America, this too seems like a logical first step choice. BlackBerry users on this corporate WiFi calling plan can switch between GSM (voice) network calls and WiFi with seamless automatic switching. The WiFi-based calls can be made from any accessible WiFi network. The service is available for a flat fee. The fee per user depends on how many people are involved.