You can make use of dynamic DNS or STUN by setting it to host:my.domain.com or stun:stun.myhost.com (where my.domain.com or stun.myhost.com are your own dynamic domain or STUN server, respectively) to do lookups as needed. This has disadvantages because it can slow things down or stop working. As usual we have already shipped the best options by default but you still deserve to know about the other options. If you do control your own routers, you can also create permanent NAT mappings that route specific traffic right to your FreeSWITCH server or phones, and the ext-sip-ip and ext-rtp-ip can come in very handy in that case. Also, if you have the means, you could use a VPN to route two independent networks to each other when both are behind NAT, assuming you can actually control the router at both ends.