Grand Central is not your typical VoIP service. While having a VoIP component, free ringback tones (that you can upload yourself), and in-call handoffs, their newest feature is sure to draw the ire of Alltel and T-Mobile.
Specifically, their newest feature is to display one number for all incoming calls to a device. Instead of getting an incoming number on Caller ID, you see your Grand Central number. This number can then be added to your Alltel My Circle or T-Mobile myFaves.
The result is absolutely free incoming calls. The two systems check Caller ID numbers against the on-list set of numbers. When they match, the call is not taken out of airtime minutes. Essentially, by having all calls forwarding from a single Caller ID number, all incoming calls become free.
Outgoing calls can also be free by either calling with click-to-call or by calling your grandcentral number from your T-mobile or Alltel phone and using voicemail call return feature.