How can I deactivate reception of SMS on FP3?

A friend of mine has FP3 and FP4 using two SIM-cards with the same number.
Her main phone is the FP4 and the FP3 is more like a backup.
Everything works fine, except that incoming SMS will show up on the seldom used FP3 only!
She wants all SMS coming to her FP4.
Is there a way to deactivate reception on the FP3 but keep its mobile phone capabilities? I.e. just removing the FP3-SIM is no option.

Regain your privacy! Adopt /e/ the unGoogled mobile OS and online servicesphone

The internet wants to tell me this is difficult to achieve without one card getting deactivated by the mobile network provider. Does the mobile network provider in question officially offer or support this?
Else I would expect technical difficulties.

Swapping the SIMs between the phones was already tried?

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The provider delivers this Multi-Card with the same number for usage in two phones. But does not explicitly say something about this SMS-issue.

We tried deactivating SMS-service for messages on the FP3. Then no SMS arrived at the FP3. But none at the FP4 either. :unamused:

Unfortunately, swapping is not an option due to technical reasons.

However, by changing the way of working with the two phones a solution to this problem is not critical anymore but only nice-to-have.