Not sure why I found it so difficult to find this tweak but I’m going to document it here in case I need it in the future or if anyone else has the same problem.
qualify_frequency in the aor section! This causes Asterisk to send OPTION requests to keep the connection alive.
The normal timeout expiration setting or keep_alive_interval setting don’t seem to apply to the UDP transports?
The topic that pointed me towards this solution is here:
thecomputerperson 
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