All vendors of System V based Unix have been paying license fees to SCO with the exception of Sun Microsystems. Sun paid SCO a big lump sum for a perpetual no-fees license right after SCO bought Unix from Novell.
IBM has Unix source code which was the base for developing AIX, so they too have probably been paying some license fees to SCO.
There is no known Unix code in Linux. SCO is mainly acusing IBM of violating "trade secrets", not code copyrights.