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:

USA415 555 2671+14155552671
UK020 7183 8750+442071838750
China021 6309 5246+862163095246
Australia(08) 9287 8230+61892878230