In the web-interface go to: Settings - Telephony - VoIP - Audio - RTPĀ
the RTP port used for RTP stream
When enabled, Maxwell will use random port from range <RTP.Port> - <RTP.Port+50> for RTP stream
Example:
<param name="SIP.RTP.Port" value="5004"/>
<param name="SIP.RTP.UseRandomPorts" value="1"/>