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howto_ims.md

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How-to Enable IMS on 999xx PLMNs

It is possible to activate IMS on PLMN IDs such as 999xx using OMNT. To achieve this, the following steps should be followed:

  1. In addition to the internet APN, create another APN value for IMS on the smartphone.
  • Go to Settings/Network and Internet/SIMs/<Name of SIM card>
  • Add the APN in Access point names. (Name: ims, APN: ims, APN type: ims, APN protocol: IPv4, APN roaming protocol: IPv4). After that, save it and leave the internet APN selected.
  1. Disable SQN checking on SIM cards. (See the Disabling / Enabling SQN validation section of the sysmoISIM user manuals)

WARNING: This step disables a major security feature of the 3G/4G/5G authentication and key agreement. It is necessary when using separate HSS instances for CN and IMS (e.g., Open5GS-HSS for CN and PyHSS for IMS). Without SQN check, there is no protection against authentication replay attacks. Only use if you really know what you’re doing, and only in a lab. It is provided here because it is recommended in Step 23 of Open5GS's VoLTE tutorial.

# Add '--pcsc-shared' flag if the card status includes 'Shared Mode'
# p: Card reader number in the 'pcsc_scan' output
./pySim-shell.py --pcsc-shared -p 1
pySIM-shell > verify_adm <your-admin-key>
pySIM-shell > select ADF.USIM/EF.USIM_SQN
pySIM-shell > read_binary_decoded # Check the "sqn_check" value
pySIM-shell > update_binary_decoded --json-path $.flag1.sqn_check false
pySIM-shell > read_binary_decoded # Check again if the "sqn_check" value is false
  1. IMS activation by overriding the IMS settings may be required if the PLMN ID of the SIM card is not in the carrier list. SysmoISIM cards can connect to IMS without needing extra SIM card configuration if the PLMN ID is 00101, but for some PLMN IDs such as 999xx, the following steps should be applied:

  • The carrier permissions should be activated. (See SIM Card Setup for Carrier Permissions)

  • Open the OMNT app and enable CARRIER_VOLTE_AVAILABLE_BOOL and CARRIER_VOLTE_PROVISIONED_BOOL in Settings/Mobile Network/Android 10 API 29 (Q). Then, press Apply carrier settings now.