My office number has an extension how can i add that?

If your mobile number has an extension, you can add it by inserting a comma between the number and the extension, as shown below: 

For example, if xxxxxxxxxx is your mobile number and 1234 is your extension number, you could enter your number as xxxxxxxxxx,1234.

The comma here denotes a pause of 2 seconds before auto-dialing the extension number.