Since 2010, the operators of commercial LTE networks have largely depended on legacy UMTS/GSM networks for widespread coverage and for the provision of speech services.
It has been essential for these operators to continue to provide what have been traditionally Circuit-Switched (CS) services for subscribers being served by LTE networks which are Packet-Switched (PS) only.
IOTAS-UK-MAR-047-D IOTAS VoLTE Presentation_111115May 29, 2015