To send SMS messages to contacts located in the United States using a Salesforce entry event in a journey, NTO should format the phone field in Salesforce as +l[phonenumber], where +1 is the country code for the United States and [phonenumber] is the 10-digit phone number without any spaces or dashes. This format ensures that Marketing Cloud can correctly identify and validate the mobile numbers for SMS sending.
Normalizing phone numbers in journey settings only applies to mobile numbers stored in MobileConnect or MobilePush demographics tables, not to mobile numbers from Salesforce entry events. Selecting 'US' as the sending country in SMS activity only affects how opt-out keywords are processed, not how mobile numbers are validated. Configuring the phone field in Salesforce as US locale only affects how phone numbers are displayed in Salesforce, not how they are formatted for Marketing Cloud. References:
https://help.salesforce.com/articleView?id=sf.mc_rn_june_2020_moc_normalize_phone_number_journey_settin
https://help.salesforce.com/articleView?id=sf.mc_moc_country_code_processing_for_opt_out_keywords.htm&t