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:
Alternatively, see below for a further example based on a Xiaomi Android phone running on Android 11.
Go to Settings > App > Manage Apps
Click on Messages
Click on App permissions, and ensure that SMS is allowed