My guess is you are being picked up by a cell tower that the phone company calls Milton. In some places north east of Oshawa a tower in New York sometimes picks up your signal and your bill says Waterport NY. Although I've not had this problem since switching to Telus from Rogers.
That is most likely what has happened. Phones are programmed by default to look automatically for any network that their phones support (if it's Rogers or Fido, a GSM network; if Bell or Telus a TDMA) and to lock in on the closest one. The best way to get around it is to check the display on the phone to see what company or network you are connected to. For best results, when you are in your home, go into your phone's network settings (usually buried somewhere in the settings options) and set it to manual. Usually you can scan for what's in your area you can connect to, and just choose that one; it will stay connected to it unless you choose otherwise.
And yes, Telus would not pick it up because they use a different, older technology in their cell circuitry - the US uses GSM technology almost exclusively, so no US pickup for you.