When a phone number starts with the addition symbol, the number following it is a country code.
Phone numbers in the United States are part of the North America Numbering Plan (NANP) and the country code for NANP numbers is 1.
To place an international call, one needs to dial the IDD prefix followed by the country code and then the phone number. You can read more about this here: How to call from Country to Country
Also keep in mind that since you are not in the US it may take a bit longer to verify your account. Just be patient.
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