In the web-interface go to: Settings - Telephony - VoIP - Audio - RTPĀ 

RTP port

the RTP port used for RTP stream

Use random RTP ports.

When enabled, Maxwell will use random port from range <RTP.Port> - <RTP.Port+50> for RTP stream

ParameterDescription
SIP.RTP.Port1 - 65535: 5004 (Default)
SIP.RTP.UseRandomPorts0 = Do not use random RTP ports
1 = Use random port from range <RTP.Port> - <RTP.Port+50> for RTP stream
SIP.RTP.UseSymmetricPorts0 = Disabled
1 = Enabled
SIP.RTP.UseRTCP0 = Disabled
1 = Enabled

Example:

<param name="SIP.RTP.Port" value="5004"/>

<param name="SIP.RTP.UseRandomPorts" value="1"/>