Problem appeared after last update I think
Thanks for help
Eric Collart
Welcome to the Community forum.
This is usually a compatibility problem between the Fairphone and your telephone provider. Please 
report this to Fairphone Support, what they most need to know is your provider name.
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As a test or even temporary workaround you might try to disable 4G calling.
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Woaw ! disabling 4G calling solved the problem !
Still no more problem after re-enabling 4G calling….
I hope it solved for long … Seems a glitch occurred during update…
Huge thanks , @Volker !
Thanks also to @urs_lesse , I opened a support ticket as advised…
Eric Collart
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Wow, that’s wonderful news! I’ll keep my fingers crossed that the fix is stable! 
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Well, the solution doesn’t survive to a reboot of the phone so it’s still a bug in the last update …
Hopefully, disabling 4G calling and re-enabling gives a temporary solution…
Thanks a lot again
Eric
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Interesting. I have no such problem with FP4 (I’m on Free in France).
I’d imagine that this is indeed about compatibility of the phone with the operator, as it was suggested above: