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:
1. For 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.
2. For smsgateway.ca, Gateway URL should be the following: http://smsgateway.ca/sendsms.aspx?CellNumber=%mobilePhone%&MessageBody=%smsText%&AccountKey=O877FT0Ny0nl4MgeOa070Guh7Ln5QMO
You only need to insert your AccountKey.
3. Sometimes it is more challenging, for example for www.twilio.com you need to give a link and credential like in example.
Gateway URL - https://api.twilio.com/2010-04-01/Accounts/Account_SID/Messages.json
Post data - From=Your_phone&To=%mobilePhone%&Body=%smsText%