"Port Forwarding" vs "Port Range Forwarding"

[abward]

...it is randomly assigned and differs between each call. So though the entire range is 30,000 ports only one port is actually in use at a time...

Why is it randomly assigned? For security purposes? Would it be possible to narrow this down to maybe a several hundred ports range, so we can minimize the number of ports we forward?

[VOIPoTim]

Why is it randomly assigned? For security purposes? Would it be possible to narrow this down to maybe a several hundred ports range, so we can minimize the number of ports we forward?

Most of the audio streams are directly from remote media gateways and don't pass through us so most is out of our control.