In reality, the Canada-US plan is just a billing thing, your phone should work with or without it.
Dig into your cellular- carrier settings, and turn off automatic; then choose a carrier from the list which populates. One of the carriers on the list may work better than others.
Also check your account, and phone, and make sure International calling is on. While it shouldn’t be an issue with North American calls, this is just a double check.
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