First, make sure you have entered the correct UL/DL speeds of each connection on the corresponding WAN setup page. This is important.
Next, create an outbound policy rule; Source = x.x.x.250, Destination = Any, Algorithm = Overflow, with WAN2 highest priority.
Finally, either create a new rule or edit the default rule; Source = Any, Destination = Any, Algorithm = Overflow, with WAN1 highest priority.
Just make sure the first rule is above the second rule.
I just want to add a few things to Tim’s recommendation:
Remember to configure QoS for SIP, you do not want data to enjoy priority when one connection is used for voice and data.
We have found that it is sometimes better to configure VOIP with failover, but to rather exclude the main VOIP connection from your data outbound policy.
It was necesarry to disable ALG for some of our SIP traffic to get a better voice quality
All of your Voice traffic will come from one IP, and most probably your connection will remain active even though no calls are made. In order for traffic to be routed over your higher priority connection after it had to make use of your secondary connection, it might be necesarry to force your session to be disconnected and reconnected in your VOIP outbound policy.