To access your PBX, please see this article http://support.neural.com.au/hc/en-us/articles/200598895-PBX-as-a-Service-Configuration
Once logged in, go to 'Extensions' then 'Add Extension'.
Set 'Generic SIP' for UAD and 'Local' for location then hit 'Next step'.
Fill in the boxes for 'Name' and 'E-mail' and hit 'Advanced Options'.
Look for the 'Call Control' box located at the bottom then set the Incoming and Outgoing call limit. (Generally limits are based on the number of phone lines of the handset you are using. Cisco SPA504 has 4 lines.) Then hit Save.
Once you have completed adding your extensions and making any other changes, single click Reload to commit them.
If you wish to make changes on the settings of the extensions, please check this article http://support.neural.com.au/hc/en-us/articles/200599015-Extension-Settings