Регистировать в dyndns и использовать зарегистрированное имя в глобальных настройках sip.conf в параметрах
externhost = hostname[:port]
"externhost = hostname[:port]" is similar to "externip" except
; that the hostname is looked up every "externrefresh" seconds
; (default 10s). This can be useful when your NAT device lets you choose
; the port mapping, but the IP address is dynamic.
; Beware, you might suffer from service disruption when the name server
; resolution fails. Examples:
;
; externhost=foo.dyndns.net ; refreshed periodically
; externrefresh=180 ; change the refresh interval