When you use our core field for mobile number, we will automatically format and validate the phone number in international format (for example, +15556667890 for a US phone number). This means that you don't need to worry about the format of phone numbers during uploads, or whatever format your activists use — we will do it for you.
We accept phone numbers in all countries as well, not just the US. When an activist is filling out a form, we will attempt to place people in the right country based on IP address. If we aren't able to locate the activist based on IP address, we will show the flag and format of the default country you selected for the group. If this is still not correct, the activist can select the appropriate country from the 'country' dropdown menu (they may have to click the 'not you? click here' text), where they can select the correct country.
When an activist is filling out a form, we will automatically reject bad mobile numbers (like 123-456-6789). If an activist enters this or another bad phone number, they will get an error message highlighting the mobile phone question and not be able to submit the form.