Deutsche Telekom plans to switch all ISDN lines to IP technology by the end of 2018. Letters promoting IP technology have been sent out since 2014, because ISDN is now technically obsolete. For Deutsche Telekom, providing ISDN technology is comparatively expensive compared to IP lines, so ISDN is to be discontinued in the next few months.
For ISDN multi-device lines, migration has already been carried out millions of times and is technically no longer a problem.
The situation seems to be different for ISDN system connections. Until recently, we still had a system connection and wanted to migrate it to IP technology. Deutsche Telekom’s product for this is called “SIP Trunk”, and Telekom recommends the use of certain IP telephone systems for this purpose.
However, we wanted to use our own open source PBX for this, which should not be a problem since Telekom has implemented the SIP protocol and has written this down in its own “technical guideline” 1TR114 and 1TR118. However, we had problems with “early media”, i.e. the caller did not hear the dial tone, only silence on the line until the call was answered.
After days of troubleshooting with protocol analysis tools, studying the technical guidelines, hundreds of test calls, and unsuccessful calls to the Telekom hotline, I wrote a detailed post in the “Telekom Helps” forum. I had noticed that of the 200 or so Telekom servers that were alternately receiving incoming calls, only certain servers were correctly transmitting the dial tone. So there were different software versions in use at Telekom, and some of them were faulty. Depending on which server was used to transfer a call, the error occurred or not. Now the technicians of the Telekom were interested in it and after about a week they got the feedback that the problem was recognized and probably exists nationwide.
Since most telephone systems from large manufacturers such as Agfeo, Elmeg, etc. do not use the “Early Media” feature, but have the call sign generated on the Telekom side, the error is probably not noticed by most customers.
Addendum 30.11.18: After 4 weeks, finally a call from Telekom: The problem has been fixed.
Addendum 3.12.18: Another call from the technical service of Telekom, the troubleshooting has been delayed and they want to inform me again.
Addendum 5.12.18: New call from Telekom’s technical service. The fault has now been eliminated.
Addendum 7.12.18: We report to Telekom that the error still exists and send further call logs. Shortly thereafter, we receive a message that baffles us: The “Early Media” feature is not supported by Telekom on the SIP trunk and is to be switched off completely soon. However, we were not told why it is documented in the Telekom interface description 1TR118 and why we were held up for four weeks with alleged troubleshooting.
