Default [Q] Trouble moving rooted/custom S3 to T Mobile

This may be a cross-forum thread, but...

So my wife just moved from Verizon to T Mobile and I gave her my Verizon S3. It's rooted running CarbonRom. She was unable to set up her voicemail, so she went in to the store. The guy there told her she's only on 2G and she'll have to unroot and go back to stock to get 4G / LTE. Does any of that sound correct?

I found the stock ROM and I'll try that first before I unroot (for restoration purposes). It just seems unlikely that the root or ROM would be the problem.