SignalWire does not publish a list of IP’s for public consumption as IP’s may change. Customers can periodically run a command, or create a service to update firewalls regularly:
dig sip.signalwire.com | egrep ^sip | awk '{ print $5 }' | xargs -n1 -I{} iptables -A INPUT -s {} -j ACCEPT
It is highly recommended to use domain-based authorizing as those are subject to change! Media comes from a wide variety of locations and is non-deterministic.