C.1 Emergency calls

23.2263GPPGlobal Text Telephony (GTT)Release 17Stage 2TS

The network has no means to distinguish text emergency calls from voice emergency calls in areas where the same number is used for both types. It may also be desired that even a phone borrowed for the purpose of making a text emergency call shall get the text service without any specific text subscription. Therefore, in order to meet these requirements, it shall be possible to configure the network to route all emergency calls through CTM-SRF server nodes.

Figure C.1: Emergency call routing

If the emergency service use the same number for emergency text calls as for emergency voice calls, routing of all calls, regardless of text or speech, to a CTM-SRF server can be accomplished by configuration in the MSC/VLRs. The MSC/VLR can be configured in a way that, depending on the Emergency Centre addresses, emergency calls are always routed via a CTM-SRF server.

The CTM-SRF links in the CTM/textphone conversion function and routes the call further according to the received IAM.

The CTM-SRF node could recognise emergency calls by knowing all numbers of the emergency centres in question.

For US E-911-calls an additional method is given by looking at the CPC field. A special value is given.

– CPC "H’E0" represents "emergency service call".

In general, the CTM-SRF server handles emergency calls in the same way as normal Mobile originated calls.