Sending SMS to a custom or static phone number is required when you want to add the static number to get specific SMS when flow triggers. You can set custom numbers for different scenarios depending on your requirements. However, the system allows you to get dynamic numbers from flow to send SMS every time.
Static SMS Example Scenario: You would like to get an SMS on your phone number every time a new form is submitted on your website. In this case, the static number stays the same for every form submission unless you change it.
Dynamic SMS Example Scenario: You would like to send an SMS to customers every time they make a purchase on your website. In this case, dynamic numbers change every time for new customers.