The Greek wiretapping case involving Ericsson equipment was not due to a “trapdoor” but a malicious implant installed by a threat actor. This is documented in an excellent article by the IEEE [1]
Darknet Diaries produced a podcast on the whole affair which makes for great listening. The podcast episode includes additional details which have surfaced after the IEEE article mentioned [2]
Some of the Nokia routers from the early 2000s made in Oregon also did allegedly.
https://www.schneier.com/blog/archives/2006/02/phone_tapping...
https://www.schneier.com/blog/archives/2006/06/greek_wiretap...
https://theintercept.com/2018/03/01/nsa-global-surveillance-...