"The wouldn't have to if the EU ruly was one common market like the US."
What about the 50 different federal laws then??
I think you are referring to state laws, as Federal law is done at the Federal level and uniform across all states. Interstate commerce is regulated at the Federal level. If laws conflict, Federal law prevails. The issue, as I see it, with cellular in the EU is the companies aren't really pan-EU but country specific in many cases, with subsidiaries in various countries. As a result, legislation is required to try to end a lot of the issues caused by geographic boundaries taht do not occur in the US market.
TMobile, for example, doesn't care if you have a NY phone number but live in California. It's still all the same Tmobile; and the same price and services.
Can you get a German phone #, with an account in Germany, and use it permanently, or even mostly, in say Portugal without hitting caps or extra charges for excessive roaming? The answer, per the EU, seems to be No:
Warning
If you use your mobile phone abroad permanently, for example if you move abroad and keep using your sim card from your home country, your mobile operator may charge you extra for roaming. However, these charges are capped under the fair use policy.
and
Monitoring roaming use
As part of their fair use policy, your operator can monitor and check your roaming use over a 4 month period. If, during this period, you have spent more time abroad than at home AND your roaming use exceeds your domestic use, your operator may contact you and ask you to clarify your situation. You will have 14 days to do so.
If you continue to spend more time abroad than you do at home and your roaming consumption continues to exceed your domestic usage your operator may start charging you extra for your roaming use. The surcharges (excluding VAT) are capped at:
- €0.032 per minute of voice calls made
- €0.01 per SMS
- €3 per GB of data (cap in 2021)
That is why I say it isn't really one common market in this case. It is not uncommon for someone to have a mobile number from an area where they do not live; often because that's been their number for years and don't want the hassle of changing it and getting a new number to all their contacts.