I have IMS war stories, but they're from when we started deploying core services up to 2008 (from pre-IMS SIP and cobbled together Asterisk and Cisco Callmanager bridges to full-on 1st gen voice core, with VoLTE coming later as we replaced the radio layer, and VoWifi as a stopgap for operators with poor coverage).

I did a lot of handset testing, but mostly on the radio and broadband side, and to be honest I wouldn't mind seeing a good, auditable OSS stack (with attestation and SBOMs) since there are so many potential security issues in handsets these days...