If you are creating a membership by copying over the data from an existing non-member record (like a joint owner or co-borrower is joining the credit union), the system will copy over all phone numbers tied to that non-member record and attach them to the new membership, then delete the old numbers tied to the non-member (since the non-member record itself also gets deleted at that time).
This is not the same when copying from another existing membership, or when copying from a closed membership in MSHIST. For those, since the original records will not be deleted, the system simply makes a copy of all numbers to attach to the new membership account number, leaving the original ones attached to the other membership or closed member record as is.