TAC issued the following commands on the Unity servers CLI:
run cuc dbquery unitydirdb select objectid from tbl_User WHERE DisplayName='8603'
No records found
utils cuc users bulk_clean_orphans mapentries
Executed command unsuccessfully
No valid command entered
run cuc dbquery unitydirdb delete from tbl_DtmfAccessId WHERE DtmfAccessId = '8603'
Rows: 1
The command completed successfully.
Went into CUC and was able to Import user now. Deleting the 8603 user resolved the problem.