• Login to the system administration
• Navigate to 'Settings: Server: Edit'
  
• Click on the 'Other Networks' button
• Set 'Mode'='With Access Code'
• Click on the 'Add Network' button
• Set 'Name'='Trunk1', for example
• Set 'Prefix'='8', for example
• Check the 'Strip Prefix' option to string the 'Prefix' from the dialed number
• Set 'Allowed Range'='2-4' to allow calls to Extensions on other networks that start with 2, 3 or 4, for example
• Set 'Hidden Prefix'='55', for example, to add '55' in front of any dialed number
• Select the desired trunk under the 'Trunk' field to which all of the above rules will apply
  
• Click on the 'Save' button
  

NOTE:

The prefix number can be up to 3 digits long

Examples:
We'll try to dial Extension '1000' on remote system with access code '8'

- With 'Modes'= 'With Access Code', dial '*188 + $PREFIX + $EXTENSION' (e.g. *188 8 1000 )

- With 'Modes'='Without Access Code' dial '$PREFIX + $EXTENSION' (e.g. 8 1000)

- With 'Modes'='With Access Code', 'Strip Prefix'='On', prefix is stripped from dialed number. You dial '*188 $PREFIX + $EXTENSION' => system dials '*188 + $EXTENSION' (e.g. *188 1000)

- With 'Hidden Prefix'='123' dial '*188 + $PREFIX +$EXTENSION' => system dials '*188 + $PREFIX + $HIDDEN_PREFIX + $EXTENSION'(e.g. *188 8 123 1000)