Enable number sync and call forwarding on non ATT branded phone

[AC Question]

Was finally able to number sync to my Gear S3 and get call forwarding working on the ATT network on a non ATT branded phone (LG V20). Store rep changed the IMEI number to the one used for a V20 branded phone on their store display table and all worked. Number forwarding from phone menu now works, number sync'ed to Gear S3 also works. Store rep mentioned that he added four zeros at end of IMEI to make it a generic IMEI. Tried everything on the web that was suggest to solve this problem, nothing worked till this.