Try removing all the special characters from the number you are using to send the SMS. Furthermore, you might want to check your SMS gateway service account dashboard - most likely, they have a log of API requests here with the exact reasons why messages may not be coming through.
You also need to check sms settings of DA server. For example, for a clicksend.com, Gateway URL should be the following: https://api-mapper.clicksend.com/http/v2/send.php?method=http&username=xxxx&key=xxxx&to=%mobilePhone%&message=%smsText%&senderid=Name_of_your_company
You only need to insert your username and key.