SIM1 Stops Working After a Few Days (No Calls In/Out)

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Device Model & MIUI Version

• Device Model: Xiaomi Redmi 9T
• MIUI Version: MIUI 14.0.2.0.SJQCNXM
• Android Version: Android 12 SKQ1.211202.001

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Bug Description​

After running my Xiaomi phone for several days while roaming within the EU, the primary SIM card (SIM1) stops functioning properly for voice calls. Specifically:

1. Outgoing calls:
   • When I attempt an outgoing call on SIM1, the phone displays “Dialing” for about 10 seconds, then shows “Call ended” and plays the call-end tone.
   • No voice transmission occurs during this interval.
2. Incoming calls:
   • The phone sometimes rings, but upon answering, there is no audio on either side (caller cannot hear me, I cannot hear them).
   • Other times, the phone doesn’t ring at all.
3. SIM2 Behavior:
   • SIM2 continues to work properly for both incoming and outgoing calls without any issue.
4. Temporary Workaround:
   • Rebooting the phone or toggling Airplane Mode on/off temporarily resolves the issue for a few days. After some time (usually a few days later), SIM1 fails again in the same manner.
5. No Alerts or Indicators:
   • There is no notification or warning that the phone is unable to make or receive calls on SIM1, making the issue hard to detect until I actively try to call someone.
6. Home Network vs. Roaming:
   • This problem has so far only been observed while roaming in the EU; I have not noticed it (or cannot consistently reproduce it) on my home network. However, I am rarely on the home network, so it is possible I haven’t tested it thoroughly there.
   • I have owned this phone for 3.5 years and have never experienced anything like this until recently.

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Steps to Reproduce​

1. Insert two SIM cards, with SIM1 set as the primary for calls/SMS.
2. Travel or remain in an EU roaming environment (with SIM1 on roaming).
3. Use the phone normally for a few days.
4. Attempt to place an outgoing call or receive an incoming call on SIM1.

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Expected Behavior​

• SIM1 should consistently allow both outgoing and incoming calls without losing voice transmission.
• The phone should not require frequent restarts or airplane-mode toggles to restore calling functionality.

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Actual Behavior​

• After a few days of normal use in roaming, SIM1 fails for voice calls (both incoming and outgoing), with no audio signal transmission and no clear error message beyond “Call ended.”

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Workarounds Tried​

1. Restart Phone: Restoring functionality for a short period (several days).
2. Toggle Airplane Mode: Also restores functionality for the same short period.

Both measures only work temporarily; the problem reappears after a few days.

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Frequency of Occurrence​

• Occurs consistently after a few days of normal operation in roaming mode.

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Additional Information​

• This issue started occurring recently; I have never experienced it in 3.5 years of owning the device.
• No related errors or warnings appear in the notification bar or Settings menu.
• Crucial note: There is no obvious indication to the user that SIM1 is malfunctioning until a call is attempted or missed.

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Request for Help​

• Solution or Update: Are there known patches or recommended settings (e.g., APN configuration, network modes) that can prevent SIM1 from dropping calls while roaming?

Thank you for looking into this. Any guidance or fix would be greatly appreciated, as this issue significantly impacts my ability to make and receive important calls without constant monitoring.