My unlocked AT&T S5 won't work with my Verizon service

[the real webgrunt]

I was given a AT&T S5 from a friend who is the original owner. I emailed AT&T for an unlock code and they sent me one. I used it per their instructions and the phone said it's unlocked.I put in my Verizon sim card from my Note 2 but it won't let me text or call. It says: "Mobile network not available. Out of service area."

I called Verizon, they had me read them the IMEI number, and after some time said I'd need to take it to a Verizon store to get a new SIM card for which there would be no charge.

I took it to the store and the guy there said that I do not need a new SIM card but if I did it would be $25. Then he tried to add the S5 to my account and it said the device could not be added but didn't say why. He said "It's probably because it's unlocked." I thought unlocked meant that it was free to be used with any network?

Anyway, I then tried to add it to my account myself online and it said "There was an error with one of our systems and we're working on it now. We're sorry for the inconvenience. Please try again in a few minutes."

I opened a chat with a Verizon rep online and they said that not all phones from other providers will work with Verizon's service.

Is there any way to make this phone work with my service? If not I'll probably give it back to him, but I'm hoping I can.

 

[AshManMB]

As far as I know, verizon uses CDMA network, and not gsm. The att phone can work on tmobile, but probably not verizon or sprint. Though I haven't checked in a long time. Been about 5 or so years since I last used verizon cell service.

 

[T79phoenix]

I just checked my phone network options (Verizon) and here are the choices. GSM is an option.

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[ironass]

The AT&T model of Galaxy S5, G900A, is a GSM phone and will not work on the the old Verizon CDMA network on 2G & 3G. CDMA is now only used, partially, in the U.S. and China,,, and even their 2 networks are not compatible.Because of the fragmented nature of the U.S. networks, the AT&T phone will only be partially compatible on some of T-Mobile (USA) bands for 3G and 4G/LTE.

 

[the real webgrunt]

...so maybe Verizon blocks adding that model because of potential compatibility issues? I guess that makes sense. I'm glad it was a gift rather than a purchase!