[Serusers] ser on wireless interface

Hi, I am using ser as a registrar/proxy server. I seem to be able to register a client on the wireless interface on the sip server only if the name of the client is included in the hosts file on the server that too the ip has to be static. Is there a way that i can have mobile wireless client register with dynamic ip on ser? Any insight would be of great help.Does ser listen to wireless clients on any particular port no. other than default 5060. Please guide. I am not able to pass voice packets between a wireless and wired client that are registered on the ser. I am stuck. Any hint would be of great help. Thanks, Annie