Diagnostic ID: 10500 and sip-reason=”Q.850; cause=44; text=”Requested Circuit/Channel N/A”

Hi all,

recently one of our Skype for Business Enterprise Voice customer found many of these Unexpected errors in it’s Monitoring Reports

Err_1

Request: INVITE
Response: 500
Diagnostic ID: 10500
Category: Unexpected
Component: Mediation Server
Reason: Gateway responded with 500 Server Internal Error
Description:
This response from a gateway can occur if the gateway encountered an unexpected condition that prevented the request from being fulfilled. If the sip-reason parameter is available, this will contain the specific ISDN/ISUP Cause code from the PSTN interaction with the gateway that triggered this response to be generated. Please refer to gateway documentation for more details.

Searching deeply with the User Activity Report, the Diagnostic Header was the same on every Unexpected calls:
sip-reason=”Q.850; cause=44; text=”Requested Circuit/Channel N/A Err_2 Response code: 500
Diagnostic ID: 10500
Request type: INVITE
Content type: multipart/alternative
Source: (removed)
Application: MediationServer
From user URI: (removed)
To user URI: (removed)
From user agent: UCCAPI/15.0.4927.1000 OC/15.0.4927.1000 (Skype for Business)
Diagnostic header:
10500; source=”<Front-End>”; reason=”Gateway responded with 500 Server Internal Error”; component=”MediationServer”; SipResponseCode=”500″; SipResponseText=”Internal Server Error”; sip-reason=”Q.850; cause=44; text=”Requested Circuit/Channel N/A“”; GatewayFqdn=”<VoiceGW>; trunk=<Trunk>”

Customer’s Voice Gateway are Sonus SBC 1000 with two ISDN E1 PRI lines (30 channels each) used to migrate users from a legacy PBX to Skype for Business EV.

To solve this issue I simply switched the ISDN E1 Signaling Group Indicated Channel setting from Exclusive to Preferred

Err_3