Emergency Calls in the IMS
Owing to national or international regulatory requirements, most of the cellular networks in the world have to fulfill a set of requirements related to the establishment of emergency calls. The IMS is not an exception, as it is a part of a cellular network.
The requirements for the support of emergency calls vary from country to country. For instance, in many European countries it is required that the user be able to make a call to the emergency center when the IMS terminal is not even provided with a Universal Integrated Circuit Card (UICC). In other words, the network has to be able to route an emergency call to the emergency center even when the user is not authenticated. In other countries the network has to provide accurate geographical location information of the user making an emergency call.
IMS Releases 5 and 6 did not provide architectural support for delivering emergency calls over IMS. Full support for IMS emergency call is provided starting at 3GPP IMS Release 7. Release 5 and 6 IMS terminals must issue emergency calls over the circuit-switched domain. A Release 5/6 P-CSCF that receives an INVITE request for setting up an emergency call will just reject it by replying with a 380 (Alternative Service) response, indicating that the emergency call should be placed over the circuit-switched domain.
Now, let us focus on the support for emergency calls in IMS, as specified in 3GPP TS 23.167 . We have just analyzed in the Chapter ...