Before trying the scripts, you may want to check "firewall optimization" is normal or aggressive. The VoIP configuration docs suggest conservative, which could be aggravating this particular problem. I've bumped mine to aggressive, but have no idea if this will cause other issues.
https://docs.netgate.com/pfsense/en/latest/book/config/advanced-firewall-nat.html#config-advanced-firewall-optimization
https://docs.netgate.com/pfsense/en/latest/book/config/advanced-firewall-nat.html#state-timeouts