If you have subscribed to SMS sending, and to ensure that SMS sending works as expected, make sure to format mobile phone numbers in the E.164 international format (with a '+' and country code).
Here are a few examples:
|USA||415 555 2671||+14155552671|
|UK||020 7183 8750||+442071838750|
|China||021 6309 5246||+862163095246|
|Australia||(08) 9287 8230||+61892878230|