SMS-Thread issue telephone number format

There is one annoying issue with SMS: In my old phone (Nokia 6230i :-)) there could be several alphanumeric characters between the numeric characters, mostly used: the slash, as common in Germany. The FairPhone (or Android?) doesn’t allow that. So I changed this for several numbers.
But: When I get an SMS from a person where I removed the slash, and I try to answer, the message won’t be sent (message: failed to send the message, red triangle icon with exclamation mark). Then I copy it to a new message, pick the contact from the adressbook, the message will be sent and this answer appeares in the same (!) thread. But next time when I get a message from the person, the direct answer won’t work again.
How can I get rid of that behavior?


There is a similar topic regarding this issue. Might be worth a read.

Yes, its worth to read, but do not solve my issue. The problem is, that the incoming SMS will be sorted in the existing thread with the unvalid phone number (this is the message, I get if I tap on the icon left, trying to edit this number, but it is not possible to edit).
If the FP would be able to ignore the slashes, it would be the best solution, because it would not be necessary to edit some thousand of numbers for the future.