Breaking NAP w/Tracking #, but using "Secondary Number" for main phone #

I spent some time with a client's SEO firm today, and they proposed something I think is likely problematic. They have changed the client's main GMB phone # to a tracking #, and have left most other citations on the old (but real) business phone number. They then put the real main line into GMB as a "Secondary" phone number in the GMB page.

They believe this is a way to circumvent NAP conventions so they can track GMB calls without changing the phone # on all directories.

Any thoughts on this? I have serious concerns, but I can't say I've tried this one.