https://www.telesec.de/assets/downloads/PKI-Repository/T-TeleSec_GlobalRoot_Class_2.cer SIP normalization needs to be used to add specific security SDP parameters that are mandatory by Telekom https://cpbx-hilfe.deutschland-lan.de/de/ratgeber-zur-konfiguration/spezialkonfigurationen...
We recently had to move from an ISDN Hand-Off to a SIP Based Service from T-Mobile called Company Flex I was able to find a guide from another post, https://community.cisco.com/kxiwq67737/attachments/kxiwq67737/discussions-uc-infrastructure/168390/1/Telekom%20Company%20Flex%20...