I finally I got a answer from fairphone:
Step 1: 3rd-party apps
is not the case, skip
Step 2: Safe mode
- Reboot your Fairphone 3/Fairphone 3+ in Safe mode
- Make a call to test if the same issue persists.
- Does the issue still occur?
- If no, follow the next steps:
- Make a Back up of your personal data
- Perform a Factory Reset
- Let us know if the issue is solved when no apps have been reinstalled.
- If yes, please, answer the following questions:
- Does it happen in every call or only with certain numbers?
- Does it happen only from a certain location or everywhere?
Step 3: SIM Cards
On several occasions, we have verified that those issues are due to the SIM Card, phone provider, and/or sim slot. To provide you further assistance, we will need to rule out these two causes because otherwise, the repair center will not be able to replicate those issues. **
Please, follow the next steps:**
- Make sure your SIM card is newer than one year.
Some providers supply it for free (an option might be to treat yourself to a new SIM card).
- If you have 2 SIM cards, remove one SIM Card and test them one at a time
- With only one SIM card
- Does the issue still happen?
- Test your SIM card in both SIM slots
- Move the SIM card to the other slot. Does the issue still happen?
- If no, let us know in which sim slot (1 or 2) the problem occurs.
- If yes, please keep reading.
- Test the phone with a SIM from a different provider(newer than one year)
- Does the issue still happen?
- If no, let us know which providers worked and which one didn’t.
- If yes, please keep reading.
Step 4:Take some screenshots
We will need the following additional information to investigate further:
a) Build number
- Go to Settings > About phone > Status >(scroll all the way down) About phone > Build number
- Take a Screenshot(s) and send it to us
b) SIM status
- Please go to **Settings > About phone > SIM status >**Select the SIM card affected
- Take a Screenshot(s) and send it to us
c) Network Provider(s) and country
If you didn’t specify it already, please let us know which network provider you are using and in which country you are having these issues.