iPhone from AT&T is not compatible with Verizon network.

If you buy a phone from a service provider it will be locked to that provider because the cost of the phone is spread across the contact. therefore you can't expect to leave that provider until you've cleared the cost of the phone. Ask AT&T if they will unlock the phone once you've paid off the $200.

Verizon uses CDMA while AT&T uses GSM technology

The Verizon CDMA model A1661* can be used on the AT&T network, yes.

The AT&T GSM model A1784* of the iPhone 7, 8 and X does not have any CDMA radios and won’t fully work on Verizon

• Model A1661*
• FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30)
• TD-LTE (Bands 38, 39, 40, 41)
• TD-SCDMA 1900 (F), 2000 (A)
• CDMA EV-DO Rev. A (800, 1900, 2100 MHz)
• UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
• GSM/EDGE (850, 900, 1800, 1900 MHz)

• Model A1784*
• FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30) 
• TD-LTE (Bands 38, 39, 40, 41)
• UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz) 
• GSM/EDGE (850, 900, 1800, 1900 MHz)

Hi Roger Wilmut1

Even if he gets it unlocked from AT&T it will not work on Verizon CDMA network. It all depends on what model of iPhone he has bought from AT&T A1661 or A1784.

I presume it may be A1784

Regards

Sravan

Thanks. One of these days everyone will get their standards sorted out...