Change phone from Verizon to AT&T network.?

Someone gave me a BlackBerry, but he used Verizon and I use AT%26amp;T. How can I change the network on the phones?Change phone from Verizon to AT%26amp;T network.?
You cannot use a Verizon phone on AT%26amp;T!!!Change phone from Verizon to AT%26amp;T network.?
Hi. You wouldn't be able to use that Blackberry unless you switch over to Verizon's service. AT%26amp;T runs on a GSM network as well as T-mobile. Verizon and Sprint runs on a CDMA. Unfortunately, phones could not be unlocked or changed to switch networks. Hope this helps!