To format mobile numbers for Email to SMS, please ensure that the numbers are in their correct international format and that you're using the correct country prefix/dialling codes.
Please refer to the article on what format do my mobile numbers need to be in? for more information.
Below is an example for an Australian mobile number:
Country Code: 61
Number: 9 digits long, excluding the leading '0'.
Example domestic number: 0447973739
Example international number: 61447973739