Can I use my Sprint TP on the Verizon network?

[Mr31337]

My girlfriend has Verizon and has a second unused line with phone number. I have Sprint, but wanted to switch my phone to her plan and use that unused phone number. The Verizon service rep that I just talked to said that I cant because Sprint is a different type of network. Is this true, or was she just feeding me some bull to try to get me to have to buy a Verizon phone?

Also, if I AM able to use my phone, what is the necessary steps in order to do that? Does it need to be unlocked or anything?

Thanks for any help.