I'm sorry, there is no way of getting a copy of the CPT ontology from BioPortal—the license never allowed for that. We do not expect it to become accessible through BioPortal in the near future.
As with several other ontologies, individuals have the ability to download UMLS themselves using their UMLS license , and use the umls2rdf conversion software  to work with it in their own OntoPortal repository  for their own purposes.
Of course those users must remain in accordance with the UMLS and CPT licenses. Perhaps this will provide a useful workaround for some individuals.