I had difficulty sending SMS messages (they took a long time to send) and incoming calls were sometimes choppy, even in areas with excellent network coverage. I can’t explain why, but changing the Mobile Network settings solved the problems :
In Settings / Mobile network :
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uncheck “Select network automatically”
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click on Network and start a search (if not automatically, use the top menu)
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in the results, select your operator (Cdiscount mob - FR 20 in my example)
Normally, there’s no need to restart the phone.
Note that Cdiscount Mobile uses the Bouygues Télécom network, so the procedure could potentially also work with the source operator, as well as operators using the same infrastructure.