This may be due to a phone setting or 3rd party texting app which prevents sending an SMS to 5-digit numbers.
Please check the permissions of your Android messaging app.
To do this go to Settings > App Manager > All Tab –>Messenger.
or Permissions > Premium SMS, then tap Always allow.
Scroll to permissions.
There should be three choices - Ask, Never Allow, and Always Allow.
If the setting is Never Allow or Ask, change this to Always Allow.
Also check if there is any 3rd party app for text, messaging or spam filters is preventing the phone from sending the SMS.
Below is some helpful info for Samsung users: