Transfer service from ATT to Verizon with iPhone 7

What model iPhone did you get exactly? Not all iPhone 7 models can work with Verizon or others on their network.

The A1660 models will work on Verizon and Carriers that use their network.

The A1778 will not unless the Spectrum is strictly GSM. If they require CDMA support it will not work.

iPhone - View countries with supported LTE networks - Apple

With that said, if you have a compatible iPhone model and if you have no outstanding debt with AT&T, and your contract has been fulfilled, then you can contact Spectrum and have them port over your number.

If you don't need to keep your number then, you can just go to Spectrum and ask them to activate your phone on their service.

If you bought an AT&T iPhone 7 it is not compatible with CDMA networks like Verizon, Sprint, and the MVNOs who use their networks. It does not contain CDMA hardware.

Only AT&T can unlock their service contract with you.