Call Distortion


dear all,

Recently i am using Grandstream UCM 6202 to divert calls through a SIP Provider (Hoiio)

but i am facing this issues
me as the caller, the call quality is good. i am able to hear the other side quite well
but for the person receiving the call, they mentioned that there is a distortion in the call.

the call route is roughly as shown

Caller > Grandstream UCM > Hoiio > Receiver

when i bypass grandstream, there is no distortion on the calls it self
so most likely it should be the grandstream configuration issues

i checked hoiio setup, the CODEC they are using is G.729, GSM, G.711A, G.711U

under grandstream, i tried to add all these 4 CODEC but i am still facing with this issues

i’ve tested on the network portion, it should not be caused by network issues

anyone know what could be causing this distortion issues?


use only one Codec, make sure that your firewall correctly manages the NAT mode in “Full Cone NAT” and check if you need to change the Jitter


The outbound portion of your call audio (i.e. how the callee is hearing you) is carried on the “upload” side of your internet connection. If you have a coax or DSL internet connection, the “upload” side is typically significantly smaller in bandwidth than the “download” side.

Could it be that something at your location is consuming most of the bandwidth on the upload side of the connection? If you’re able to test after hours, you could unplug everything from the router or switch with the exception of the UCM. Then test. If the distortion goes away, then you may have found your culprit.

I had a similar problem in a small office with only a couple of phones and a DSL connection without much bandwidth on the upstream side. Turned out the problem was a PC that had a virus and was constantly uploading data to the nefarious author/owner of the virus.


this does not happen if you apply QoS correctly on the router/firewall