RTP port range in relation to Quality issues

In the FreePBX docs, I came across info regarding keeping RTP ports within a certain range otherwise it can lead to call quality issues. This is the only reference I found in the internet that says RTP port range can cause quality issues. Does anyone know why? I’ve copied and pasted the paragraph below:

“A range of 10000 ports available for RTP Media is often unnecessarily large for most small systems, because one call requires only 4 active ports. Thus, you might consider narrowing the range of ports used for RTP Media. If you do narrow the range, keep the range somewhere within 10000 to 20000 (i.e. don’t select 43500 to 44500), as going outside this range can lead to call quality issues.”


Any insight you can provide would be great.

I wrote that language, because I found that when I changed the port range outside of 10000 to 20000, I had call quality issues. Lots of audio popping, and delays in audio starting after I answered a call. I have no idea why that happened, but it did. I assume (someone correct me if I’m wrong) that my ISP or somewhere on the net, a decision has been made to give priority to UDP packets in the 10000 to 20000 range, or that there was some other conflict in the 40xxx range that I don’t know about.

Why don’t you try it yourself and report back?