Can you use an verizon iphone 4 on atts network?

I know verizon uses CDMA and ATT is GSM but is there someway to use the phone on the other ones carrier? Or is software built into the phone so its ot possible.

It's impossible i've tried

Cdma iphone 4's don't have sim trays i think if it was a 4s buy the new gevey sim from applenberry.com the one thats says cdma wcdma.