When you are using a phone number for call or sending SMS, you have to make sure, that it has been properly formatted. While International Call Prefix usually works, it's still recommended to use the Country Calling Codes (E.164 format).
A number using the E.164 format includes:
[+][country code][area code][local phone number]
Example: USA
Local phone number: 415 123 1234
E.164 format number: +14151231234
Example: UK
Local phone number: 020 1234 1234
E.164 format number: +442012341234
Example: Lithuania
Local phone number: 8 601 12345
E.164 format number: +37060112345
Using the E.164 format ensures the best delivery across the globe. Make sure to check the list of Country Calling Codes!