When someone calls your main reception number and the receptionist dials an abbreviated number like 101 to transfer the call internally to say, the accounts person, this is abbreviated dialing. Some companies use a generic naming convention like 101, 102, 103 etc, while other companies choose to use the last 3 digits of the persons direct indial number, eg. 691, 692, 693 etc.
Abbreviated dialing is configured in the cloud (contact virion support) if you have a cloud based PBX solution. If you have a PBX device in your office like the SPA9000 or a PBX from another vendor, then the configuration of abbreviated dialing is performed on that device (speak to your telephone technician).