A single user in a group text is getting each new text from a different number or in a conversation thread

Hi,

I work in sales, and we recently setup our crew for new-lead notifications using an email-to-sms gateway. Everyone else on the "group text"-list receives the notifications from the same, singular number. However, only one user receives each new message from this group text from a different number / in a different conversation thread.

This user has an iPhone 14 Pro Max, and his carrier is AT&T. 

We've reached out to our CRM provider for initial troubleshooting. We've confirmed that the originator of the group text messages should be from one number, so we are unsure why their notifications are sending from multiple numbers to this specific user. We also added other users with an iPhone 14 Pro Max and AT&T to the notification list, but they didn't experience any issues (they all got the group texts from the same number / in the same conversation thread). We've tried switching the problem-user's gateway from SMS to MMS, but that did not help... 

This user needs text message notifications for new leads, but it's hyper-annoying for these messages to come from different numbers / in different conversation threads. I am at a loss how to troubleshoot this further... Our text notifications are sending from one number, so how on earth is it separating into different numbers for just this one user?? 

Our CRM provider is confident this is a "carrier" issue, but I don't know how to troubleshoot "carrier" issues. Is there a setting in the iPhone 14 or maybe in an AT&T app that's causing his group texts to send from different numbers?? 

Any help would be great!

Thanks!