Voice over LTE is the service provided to support voice and SMS over IMS.

This page describes how to test VoLTE, using Amarisoft EPC and IMS server. It explains how to set APNs on your UE, how to configure your network to register your device and how to check your device's connection. 

IMS (IP Multimedia System) uses SIP (Session Initiation Protocol) to control all voice, video and message applications. Several server/proxy components, called Call Session Control Function (CSCF), are used to process SIP packets : 

• P-CSCF (Proxy CSCF) : first point of contact for a user. It is assigned to an IMS terminal before registration, and does not change for the duration of the registration.
• I-CSCF (Interrogating CSCF) : entry contact within an operator's network for all connections destined to a subscriber. It also forwards SIP requests or responses to the S-CSCF.
• S-CSCF (Serving CSCF) : central node of the SIP server. It is also responsible for handling the registration process and downloading user information and service profiles from the HSS.
• HSS (Home Subscriber Server) : user database containing all subscribers information to handle calls.

When registering an UE on IMS, the following identifiers are assigned :  

• IMEI : identifier for the device with the SIM card input
• IMPU : user's identifier over IMS (telephone number, SIP). Used for user-to-user communication.
• IMPI : used to identify the IMS user's subscription, assigned by the mobile operator.

First, you need to check if VoLTE can be enabled on your device. While the exact paths to the menu will depend on the UE device used, make sure that the VoLTE option is ticked in your Mobile Network settings. 

If the VoLTE option is not available, you might need to access hidden menus to activate IMS and VoLTE. On Android phones, you have the possibility to do this by dialling *#*#4636#*#*. Nevertheless, some phones only enable VoLTE with certain mobile operators and UICC cards.    

You also need to configure two PDNs used by Amarisoft MME service : 

• Internet APN
  • APN type = internet,default (or default depending on UE)
  • APN = internet
  • Name = Internet
• IMS APN 
  • Name = IMS
  • APN = ims
  • APN type = ims

Turn on Data Roaming. Save both APNs and only select  the Internet APN. If you already have another APN registered, you might have to delete it. 

If you are not using Amarisoft LTEIMS and LTEMME running on the same PC, edit the file config/ims.cfg to set the address of the SIP interface.

To define your UE in the UE database, edit mme/config/ue_db-ims.cfg.

Parameters such as IMSI, OPC and Key (K) should be given from your SIM cards' provider. Enter those values in the configuration file. 

If the IMPI is not provided beforehand, it can be easily derived from the IMSI number. IMPI is composed of the username part and the domain name part. The username part corresponds to the whole IMSI string. The domain name is composed of the MCC and the MNC. For a 15 digit IMSI, take the first 3 digits as the MCC and the two following digits preceded by a 0 as the MNC. The domain name is then in the format mnc<MNC>.mcc<MCC>.3gppnetwork.org. Finally, the IMPI is in the format username@ims.domain.

The IMPU is in the format sip:IMSI. You can assign a phone number as well, in the format tel:number.

For example :

• IMSI = 012340111111111
• MCC = 012
• MNC = 34
• Domain Name = ims.mnc034.mcc012.3gppnetwork.org
• IMPI = 012340111111111@ims.mnc034.mcc012.3gppnetwork.org
• IMPU = sip:012340111111111@ims.mnc034.mcc012.3gppnetwork.org
• tel:0600000001

If you are using Amarisoft Test SIM cards and want to test VoLTE with more than two phones, you need to create an IMPU associated wich each device's IMEI as follows, with the values corresponding to your device's IMEI. The IMEI can be found on your phone,.

impu: [
  "001010123456789",
  { impu: "tel:0701", imei: "359027072930540" },
  { impu: "tel:0702", imei: "356853053276500" },
  { impu: "tel:0703", imei: "359027072637360" },
],

Start the LTE service. In the IMS window, check the output of the users command, which should list the UE database.

To make a call : 

• MT (Mobile Terminated) Call : type mt_call <impi> in LTEIMS screen
• End-to-end call : directly call from a registered UE to another phone's IMPU tel URI

You should be able to make a call between devices registered on your network's IMS.