At&T Service and the North East Corridor are a contradiction in terms
I live between PHL and NYC and had AT&T service for years, I was always being told the service would get better. I found it hard to understand why service would be so bad while riding amtrak between NYC and Washington but it was. Most of the time calls would be dropped or hearing each other was difficult. I finally dropped AT&T and switched now I have no problems on Amtrak or even in the NYC subways.
I have always said get the best coverage for the areas you make the most calls in and do not worry about the name of the supplier.