SMS/MMS sends in Journey Builder require a fully normalized phone number — country code plus the local number with no additional formatting characters — present directly in the entry-source data extension at the point the SMS activity executes; Marketing Cloud does not automatically infer or append a missing country code on the platform ' s own initiative. Because the root cause described is that the Salesforce-sourced mobile number field lacks the country code altogether, the fix must occur at the data source: formatting the phone field value in Salesforce to explicitly include the country code prefix (+1 for United States numbers) before or as it syncs into the journey ' s entry data extension, ensuring the normalized format Journey Builder requires is present from the start. There is no native ' Normalize Phone Number ' toggle in Journey Builder settings (option A) that automatically fixes missing country codes — this is a fabricated distractor. Configuring a Salesforce field ' s locale setting (option B) affects display/input formatting conventions within Salesforce itself, not the actual stored digit string synced to Marketing Cloud. There is likewise no ' sending country ' selector within the SMS activity configuration (option C) that retroactively corrects malformed source data. Reference: SMS/MMS Activity Phone Number Requirements, Journey Builder Salesforce Entry Sources.