If you call an internal number that is engaged, the system can automatically call you back when the line becomes free.
If you have called a number and find it busy:
Press the Recall button to receive a dial tone and dial #1. You will hear a long and short dialling tone to let you know that you have connected successfully.
Replace the handset
When the extension becomes free your phone will give one long continuous ring
Pick up your phone and the extension you dialled originally will be called
This can also be used on no reply from an extension. The next time the extension is used the system will call you with one long ring. Pick your phone up and the system will call them back.