Go to Configuration->Signaling->SIP Trunks and add the two or more trunks you wish to use in the load balancing.
Go to Configuration->Routing->Load Balancing and click on the SIP Trunk List tab. Then click the Add button at the bottom.
Give the trunk a name and click Add.
Ensure the list is enabled and click Save to create the list.
Next click the Add button to add the first trunk to the list.
Select the SIP trunk from the drop down menu, then provide a weight. Here we use a weight of 1 because all trunks will have the same weight to keep the usage the same. Click the Save button once complete.
Now the first trunk will be in the SIP trunk list. Click Add to add the next trunk into the list.
Select the next SIP trunk from the drop down menu, then provide a weight. Once done click the Save button.
Now the list is complete as it has both of the SIP trunks. If you are adding more SIP trunks then verify all of them are added at this point before proceeding.
Below is your typical route to a SIP trunk. The difference is the action is "Bridge to Distributor" because this is our load balancer. Next the list made previously is selected. This dial plan now will load balance between the SIP trunks.