Audiocodes T301 timer causing 603 Decline on LYNC

I Recently had a strange issue on a customer site, customer reported they suddenly could not call some clients in Austria. And after digging into the problem we found that also som areas of Sweden and also Italy was impossible to dial via PSTN.
The customers LYNC 2010 server was connected to PSTN via and Audiocodes Mediant 1000 6.2x and the E1 ISDN trunk was working fine for most international and all local numbers.
When turning the LYNC traces the 603 DECLINE response was found.
In the AC syslog tool I found that the carrier send a “CALL RELEASED”
This is what I found in the syslog

pstn send –> PlaceCall: Trunk:0 BChannel:2 ConnID:1 SrcPN=xxxxxxxx SrcSN= DstPN=0043xxxxxxxxxx DstSN= SrcNT=0 SrcNP=0 SrcPres=0 SrcScrn=0 DstNT=0 DstNP=0 ServiceCap=M RdrctNum= RdNT=0 RdNP=0 RdPres=0 RdScrn=0 RdRsn=-1 Excl=1 Display= IE= UUIE=0, RawData:0 CLIRReason:-1 OrigPN= OLI=-1 OffhookInd=0

 pstn recv <– CALL_PROCEEDING Trunk:0 Conn:1 BChannel:2  callhndl:0 Loc:-1 Des:-1

 pstn recv <– CALL_RELEASED Trunk:0 Conn:1 RetCause:104 NetCause:16

Now the fun began, because after corresponding with the carrier (TDC) they told me, that the Call_released is send corresponding to the CANCEL that we send them.

So the gateway was sending PSTN commands without showing them in syslog – interesting. Anyway the tech informed me of the error code attached to the CANCEL message was T301 timer error code 102 Recovery on timer expired.
Okay, but the T301 setting In Audiocodes is controlled by the “PSTN alert timeout” setting found under Configuration tab > VoIP menu > PSTN Settings sub-menu > Trunk Settings – and this was set to default at -1 (which according to documentation is 180 seconds – FYI the CANCEL apparently was send after 4 seconds.

I then tried to adjust the PSTN alert timer to 300 and reboot the gateway – and HEY PRESTO – now the customer could dial the customer in Austria with no problems.


  1. Juan Carlos Rodriguez

    Hi, on March 22, 2017 I started to find the same problem in a Audicodes Mediant 1000 version 6.80A.231.002 in Spain, with Movistar carrier.
    I changed the parameter according to your recommendation and it was solved.
    Thank you very much for your help

  2. ├żorgeir j├│nsson

    Well done. first time tuching this box and this was not an obvius finde. been debugging this issue for couple of days. we saw that every other call was failing til tdc danmark and increasing this value did the trick.

Leave a Reply

Your email address will not be published. Required fields are marked *