For creating a sip trunk between didforsale and your freepbx system, first create a sip account from your didforsale account. For creating the sip account, login to your didforsale account, go to MANAGE ENDPOINTS, click MANAGE SIP and then click Add New SIP Account button. Fill the details and click add. This will create a sip account in your didforsale account. You can use this sip account to create the sip trunk between didforsale and your freepbx system.
For adding the SIP account to your freepbx system, login to your freepbx, go to Connectivity, click trunks, then click Add SIP Trunks
Inside the Outgoing Settings , add the below parameters in the PEER Details box
secret=sip account password
Also add the register string as shown below
Save and Apply Config
For making outgoing calls using this trunk, go to Outbound Routes and create a new route. Selet the trunk for this route as the newly created sip trunk