Volte IMS registration issue on CBRS 315010 PLMN - Issue making phone calls to Open5gs via Nova 430H

 

 

herlesupreeth

on Oct 26, 2023

MaintainerAuthor

NOTE: The above requirement of NAS encryption and User plane encryption is not necessary when testing with 001 01 test PLMN. The only requirement is the availability of IMS and SUPI concealment. i.e. skip NAS encryption and User plane encryption mentioned above iPhone blacklists SIM's ICCID from connecting to 5G SA if the PLMN is not one of the PLMN assigned for private mobile network Luckily, the above blacklisting can be circumvented as follows: First program the SIM with PLMN corresponding to a private mobile network PLMN e.g. 999 70 Start the 5GC with the SIM programmed PLMN Start the gNB with the SIM programmed PLMN Put the SIM in iPhone and attempt to attach to gNB. Enable 5G Standalone option in Voice & Data settings menu. Wait for UE to attach to network Then, bring down the network and put phone in airplane mode and pull out the SIM Re-program the SIM to any desired PLMN e.g. 001 01 Now, iPhone allows attaching to 5G SA network with non-private mobile network PLMN When testing with test PLNM (001 01), iPhone 13 pro was able to connect to default and ims APN and register with IMS successfully. You would need to use the following deployment file https://github.com/herlesupreeth/docker_open5gs/blob/master/sa-vonr-deploy.yaml to have a successful connection

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No smartphone natively supports automatic out-of-the-box IMS registration on a private CBRS network unless it is explicitly configured via an enterprise MDM profile or manually overridden by the user. [1, 2]

1. Apple iPhone (The Most Reliable for Enterprise)

2. Google Pixel (The Most Flexible for Labs)

3. Samsung Galaxy (Strict Carrier Profiles)

4. Enterprise-Grade / Rugged Android Devices

Key Requirements Checklist for the Core Network (Open5GS)

https://github.com/magma/magma/issues/15393
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7/28/2026: I did more research and it looks like phones on CBRS network does not initiate IMS registration. 
Solution: I need to find a phone that initiates IMS registration on 315 001 PLMN.
               The chat bot also recomended to check the sim card configuration, basically compare the 001010 test PLMN that works with CBRS 315001 that does not.
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Baicells is basically telling you the phone is not attempting VoLTE, and instead is trying to place the call using CSFB (Circuit-Switched Fallback) — which cannot work in your setup with CBRS + Open5GS.

Let’s break their message down piece by piece and translate it into plain terms.

1. “I checked the eNB Wireshark packet and see ESR (Sent by UE IMSI:5481)”

ESR = Extended Service Request

This is an LTE signaling message sent by the UE (phone) to the eNB when:

• The phone wants to make or receive a voice call

• VoLTE is not available

• The phone requests fallback to a circuit-switched (2G/3G) network

So this line means:

Your phone is telling the network:
“I want to make a call, but I don’t see VoLTE, please hand me off to 2G/3G.”

2. “The call is CSFB not VoLTE”

This confirms the above.

CSFB (Circuit-Switched Fallback)

• Requires a 2G or 3G core network (MSC)

• LTE is only used for data

• Voice is handled by legacy circuit-switched infrastructure

Why this is a problem

• CBRS LTE is LTE-only

• Open5GS is LTE/5G packet-switched only

• There is no 2G/3G network to fall back to

?? Result: The call will always fail

3. “Please check your core network and Phones”

This is Baicells politely saying:

The eNB is working correctly.
The issue is VoLTE is not properly configured or supported in:

• Your core (Open5GS)

• Your IMS / SIP setup

• Or the phone’s VoLTE profile

What this means for your call flow

Your current call attempt looks like this:

UE → LTE Attach → Dial Call
      ↓
UE sends ESR (CSFB request)
      ↓
No 2G/3G network exists
      ↓
Call fails

What must happen instead:

UE → LTE Attach
UE → IMS Registration (SIP REGISTER)
UE → VoLTE SIP INVITE
IMS ↔ Open5GS ↔ SIP server

Root causes (most common)

One or more of these is true:

1. Phone does NOT see VoLTE as available

• VoLTE disabled in phone settings

• Carrier profile missing or incorrect

• CBRS PLMN not recognized as VoLTE-capable

2. IMS is not fully configured

• SIP server reachable but:

  • No proper IMS APN

  • No P-CSCF advertised

  • Missing DNS records

• Open5GS not integrated with IMS correctly

3. Phone not provisioned for VoLTE

• Many phones refuse VoLTE on private LTE

• Some require:

  • Specific MCC/MNC

  • Root access

  • Custom carrier config

How to confirm this yourself

On the UE:

• Check VoLTE / LTE Calling is ON

• Check IMS registration status (often hidden menu *#*#4636#*#*)

On the eNB / core:

• In Wireshark:

  • ? ESR = CSFB attempt (bad)

  • ? SIP REGISTER / INVITE = VoLTE working

• In Open5GS logs:

  • Look for IMS APN attach

  • Look for SIP traffic to P-CSCF

Bottom line

Baicells is saying:

Your network is data-only right now.
The phone is trying to make a legacy voice call (CSFB), but CBRS + Open5GS only supports voice via VoLTE, and VoLTE is not active in your setup.

If you want, tell me:

• Phone model(s)

• MCC/MNC you’re using

• IMS/SIP server (Asterisk, Kamailio, etc.)

and I can help you pinpoint exactly why VoLTE isn’t being triggered and how to fix it.