Official comment

If the SMS number is imported or entered with a properly formatted country code the number is saved in that format. If your Contacts use a country code we will accept it.
If the number is entered without a country code, ONTRAPORT will format it based on the following criteria:
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The contact record's Country field, as shown in the image above. This would be if you include the Country field on your form. Note that, when we can, we will use the Contact's IP address to set the country field, but that does not always work.
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If there is no Country set in the contact record with the form fillout, we use the "Contact Owner's" country setting. This allows you to automatically format the SMS numbers of contacts assigned to a regional salesperson or business division. See "Setting the Contact Owner's Country Field" below for more information.
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If no country is specified in either the contact record or the Contact Owner's information the number will be formatted in the US / Canada format with a +1 country code.
If you are getting the "+1", and you are not in the US, follow these steps to make the default country code your own country code:
Setting the Contact Owner's Country Field
The Contact Owner is one of the ONTRAPORT Users; if you only have one login or user, it is you (also called the Administrator). To edit the User information, click on the email address at the top right of the screen, then select Manage Users.

Click each User and click Business Country to edit the country field.

If the country setting is in place in the "Contact Owner's" account (also the Admin account if you only have one log in for your ONTRAPORT account), then the numbers will default to that country code IF there is no country specified and the IP address cannot give us the country.
Manually entering or importing the SMS number without a country code will trigger the addition of the country code under these rules. Numbers already formatted with a country code will be added or imported as is, and will not be changed. Contacts who opt in via SMS, by sending a text message to your ONTRAPORT SMS number, will have the country code associated with their opt-in number.