srvcc call flow

 What is SRVCC call flow? 

In LTE network SRVCC is the feature by which a UE can roam from EUtran to Utran (vice versa) and other networks without breaking an ongoing call.  The LTE (VoLTE) is a packet-based network while UTRAN or GERAN is a circuit-based network for voice calls.

In this tutorial, we will describe the SRVCC call flow from E-UTRAN to the UTRAN.  All the details are as per 3GPP specification ETSI TS 123 21.

What is the SRVCC call flow trigger point?

The trigging point is, when the UE sends a measurement report to the eNodeB, that the cell in other RAT type is more suitable.  After that eNodeB instruct the MME for handover required. After that MME instructs the other network components for SRVCC procedure.

Before an SRVCC call flow from EUtran to Utran, UE attaches to the HSS over Diameter protocol after the call flow the UE attaches to the HLR over the ss7 network. For the data, the UE moves from PGW to GGSN.

SRVCC call flow procedure:

This section details all steps in the SRVCC call procedure.

  • UE sends a measurement report to the eNodeB.
  • eNodeB sends a Handover required to the MME for the UE.
  • MME sends PS to CS requests to the MSC server over the Sv interface.
  • MSC server sends prepare handover to the target MSC.
  • The target MSC sends relocation on handover requests to the target RNC in UTRAN (BSS for GERAN).

After the above handover sequence, there will be a circuit setup between the MSC server and MSC. Now the existing voice can use this path.


What is MSISDN or Phone Number? How to check phone number from SIM?

MSISDN full form is Mobile Subscriber – ISDN number. It is of a maximum of 15 digits long. Uniquely identifies a mobile subscriber across the globe. In simple term, an MSISDN is the phone number digits along with other information. The phone number Digi length varies from country to country.  A mobile operator gets its MSISDN range from the standard gsm body. On HLR, for a subscriber configuration, IMSI and MSISDN are the primary keys for a subscriber record. A subscriber can get the phone number or MSISDN from the device.

HLR allocates an MSISDN to a sim card on a location update of the GSM operation. Generally, this remains the same, but the mobile operator can change it. For a mobile terminated call, the GMSC sends the SendRoutingInfo GSM MAP operation with the dialled MSISDN digits. If a subscriber is online and allowed to received the call, HLR returns SendRoutingInfoResponse is with the MSRN and roaming MSC/VLR information.

Over the mobile network, an MSISDN carries information along with dialled digits. E.g., If a number is national or international.  The format of MSISDN is in E.164 over the network, according to ITU-T. The additional information helps to decide the routing and other things. For example, if a number is of type international, the first few digits are the country code, and the call will be routed outside the country if its for another country.

MSISDN Format : 


CC or mobile country code is 1 to 3 digits long. CC Signifies the country, the number belongs.   When mobile a subscriber dial a mobile phone number with the national format, the country code in MSISDN is not present. There may be a zero in place of country code.

NDC or National Destination code: Identifies the area of a network operator, in a country.

Subscriber Number (SN): Subscriber digits. The mobile operator chooses these.  The figures append after CC + NDC.

Msisdn vs Imsi:

In another tutorial, we have mention details about IMSI, which is another global identity for a subscriber.  Both can we used to route messages towards the home network of the subscriber. But there are differences. The following are the details for MSISDN vs IMSI.

  • The IMSI is embedded in a sim card while a mobile operator assigns MSISDN when the SIM registers on the network.
  • IMSI is of fix length while MSISDN length varies. The MSISDN length depends on the number of subscribers a mobile operator wants to serve.
  • IMSI has MCC and MNC codes, while MSISDN has CC and NDC.
  • Same SIM card can be assigned another MSISDN, but to change IMSI, subscriber has to change SIM card.
  • To call a subscriber, MSISDN digits are dialled, not the IMSI.

How to check phone number from sim: 

When a new sim card is purchased, the box has a printed phone number. It takes some time to memorize the phone number. But meanwhile may require to know what is my phone number from the device itself. The following are the way to know MSISDN or phone number.

What is my phone number on android:

These days android devices are ubiquitous. The phone shows the phone number in various settings.  The exact steps may differ for different models of android or os versions.

Find phone number from about phone: 

Unlock the android device. 

Go to settings .

Scroll down to the about phone.   In about phone you can see the phone number or MSISDN. For the dual sim card, the about phone on android displays the mobile number for each sim card.

Find phone number from sim card manager:

Unlock the android device. 

Go to settings.

Got to connections.

Scroll down to SIM card manager. 

In sim card manager you can see each sim card entry with mobile number with the mobile operator name.

USSD code to get the phone number:

Each operator provides ussd codes for checking balance or other things. Most of the time the ussd response contains the mobile number. E.g on Airtel India if *121# is dialed. The response have mobile number along with the ussd menu.

WhatsApp to get the phone number.

It is a very common application on a android devices. Fetches the phone number or msisdn from the phone. Following are the steps to get mobile number. 

Open the WhatsAapp 

Go to settings, This will display the profile with name and other options. 

Click on profile, there you can see the phone number.

Get number grammatically on android:

The android  provides platform for develping mobile applications. The applications are developed in java. There are inbuild librarirs for accessing andriod devices from the java code. Following is the code to get mobile number form tha java code on android.

TelephonyManager phoneInfo = (TelephonyManager)mAppContext.getSystemService(Context.TELEPHONY_SERVICE);
String mobileNumber =phoneInfo getLine1Number();
The required permission also needs to set for the mobile application.

What is my phone number on the iPhone:

Iphone also provides the capability to get the own phone number from the device. 

Go to the phone screen. 

Go to the settings 

Go to phone option. This will display the phone number.