"The specified new phone_number already exists" when trying to update user's phone number using Management API

Hi!

I have a problem with updating users via Management API. The endpoint I am using is:
PATCH /api/v2/users/{id}

There is one phone number which I cannot set as a new phone number. When I search the users in my tenant, it is not present, but the error states it already exists.

I can set some other phone numbers, it is just this one that is making a problem currently.

When I search users, there is this note: if a user was just created/deleted, it takes some time for it to be indexed. But this phone number was used 48h ago and the others that I can successfully set were used just a few minutes ago and they don’t lead to an error.

I will try to explain the situation like this:
Current phone number: A
New phone number: B
200 OK

Current phone number: B
New phone number: C
200 OK

Current phone number: C
New phone number: B
400 error

Current phone number: C
New phone number: A
200 OK

Current phone number: A
New phone number: C
200 OK

Current phone number: C
New phone number: D
200 OK

Current phone number: D
New phone number: B
400 error

and so on. So only number B is marked as existing.

Can you please help me with finding the cause of this issue?
Thanks in advance