Certain service providers request a special character in the dial string (e.g. '+'). To create a trunk that registers with the same special character:

• Navigate to 'Trunks'
• Edit the preferred Trunk
• Click on 'Advanced Options'
• Set 'Register Suffix'='+'
• Click on the 'Save' button

Now every number dialed will have a '+' added in front of it. For example, if the user dials '55510205', system will call '+55510205' when using this trunk.