If one of your customers do not receive SMS messages (appointment reminders, proposals, vehicle ready sms'es etc.) please use this guideline to troubleshoot the issue:
Check if the phone number is correct and is a mobile number.
Check if the SMS message is actually triggered. You may have SMS communications disabled for the customer or a particular appointment. Click edit on contact page to check if customer is setup to receive sms messages and click edit next to the appointment on the project page to check if the appointment has reminders enabled.
Check if the customized SMS content includes any marketing phrases or shortened links that may have caused Carriers to detect it as SPAM and filter it. If you're using a custom link in any of your customized SMS contents, make a page on your website domain for that use case instead of using URL shorteners.