Yes. 

Variable length extension dialing or abbreviated dialing is supported as long as the extension is assigned within your company.  Extensions can be from 2 to 6 digits in length.  The last 4 digits of the user's telephone number is automatically assigned to each user.  Site-to-site extension dialing is also supported provided that extensions do not overlap. 

Exceptions: 

 The user's extension cannot conflict with special services numbers.  As a result, the following numbers cannot be assigned as extensions:  00, 011, 211, 311, 0311, 1311, 411, 0411, 1411, 511, 611, 0611, 1611, 711, 811, 911, 0911, 1911.

 If the last 4 digits of the user's telephone number conflicts with a 4-digit special services numbers, the extension will be left blank when the user is provisioned.  If an extension number is required, any nonconflicting 2 to 6-digit number can be assigned by the site administrator in the My Site dashboard.

Note:  all Cloud PBX devices expect a minimum of 10 digits to be dialed before the call is launched to the Cloud PBX servers.  As a result, there may be a 3 second delay between the time the last digit of an extension was dialed and the time the call is launched.  There are two ways to avoid this delay:

 dialing a # after the extension number

offhook the phone after the number is dialed (if applicable)