Your phone should not be interfering with a US tower, as PM still does not have roaming available, and therefore should not even see the US tower.
i had a problem calling a number in my area code, but a different exchange, still same city. I had to get PM mod to have the tech team allow the number to work, as it was the most local call I could make.
The exchange is the second set of numbers. For example, 226-922-XXXX is the area code and exchange for a number in my city. 226-227-XXXX is another number exchange for my city.
these are not long distance calls, however I had to ask PM to fix the problem for me.
as I read your last message, it appears this isn't your particular problem, as you say it's intermittent.