Thank you for your reply and suggestion. I forgot to mention that I did the check with cellular carrier before posting my issue and they confirmed the result of all my previous checks: there was no call forward active and they could not do anything else.
The problem is now solved. I asked for apple assistance, they checked the phone and the only thing they could see was that the status of my battery was below 80%. After the battery was changed I activated and deactivated the call forward (done several times before without success), but this time it worked. Perhaps it was a coincidence or removing the battery somehow helped or……, but issue is solved and I am now 100% happy, like the capacity of my new battery. ;-)