When I was a kid, Portland, Ore. outgrew area code 503 and was issued an overlay area code, 971. To try to prevent confusion, the phone provider, which I believe was still USWest at the time (later Qwest later CenturyLink) made the area code mandatory. All the sudden dialing the 7 digits on most advertisements got you a recording: "Ten digit dialing is now required in the Portland area..."
Many years later, a friend borrowed my cellphone in an area where phone numbers were often dialed as seven digit. Not thinking about my being from out of state, he only dialed seven digits, and happened to get some rather confused person back in Portland. I had no idea my cellular carrier would even allow you to do that!
So overlay area codes can combine with number portability to have some odd results. A number may or may not be considered valid by the carrier, and may or may not get you who you expected, and in edge cases this may depend on the carrier.
Many years later, a friend borrowed my cellphone in an area where phone numbers were often dialed as seven digit. Not thinking about my being from out of state, he only dialed seven digits, and happened to get some rather confused person back in Portland. I had no idea my cellular carrier would even allow you to do that!
So overlay area codes can combine with number portability to have some odd results. A number may or may not be considered valid by the carrier, and may or may not get you who you expected, and in edge cases this may depend on the carrier.