IMS Release 11 (TS 23.228):
https://www.etsi.org/deliver/etsi_ts/123200_123299/123228/11.10.00_60/ts_123228v111000p.pdf
Interworking between IMS and CS (TS 29.163):
https://www.etsi.org/deliver/etsi_ts/129100_129199/129163/08.09.00_60/ts_129163v080900p.pdf