Hi,
When you write host=dynamic it means that the end device should register to you (the server). As you are the client and not the server (in cse of sip providers), you need to send the registration request (the sip register command).
As for the dynamic ip (the public ip of the server) in a NAT environment, you should always update your external ip in the sip nat settings if it changes in order to avoid one way audio problems.
Thank you,
Daniel Friedman
Trixton LTD.