billme - if you have been billing for these providers than surely you know the answer to this question: who is the billing entity? Whose NPI number have you been putting in Box 33a of the CMS 1500 Form?
You either have a legal doctors group - in which case, the group is formally constructed as a legal entity and has its own NPI Number which is used in Box 33a of the CMS 1500 Form, or each doctor is his own billing entity - and each doctor places their own NPI Number in Box 33a of CMS Form 1500.
Next, find out how the sub-contractor is going to be paid. Will he earn his money by the insurance companies paying him directly (that would mean he is the billing entity), or will one of the providers pay him (he is an employee of that provider) or do you have a doctors group with its own NPI Number and the doctors group will pay him (he is an employee of the doctors group)?
It really is that simple. Either the subcontractor is going to be his own billing entity, or he is an employee of a billing entity.
Only if the subcontractor is the billing entity would you put his information in Box 33 of the CMS 1500 Form. If the subcontractor is an employee (paid either by the doctors group or by one of your providers), then the hiring entity/billing entity's information goes in Box 33 of the CMS 1500 Form.
The billing entity's Type 2 NPI Number (who gets paid) goes in Box 33a of the CMS 1500 Form. If you are billing Medicare, you must also place the Type 1 NPI Number (who did the work) of the person who did the actual work in Box 24J of the CMS 1500 Form.
If your subcontractor is the billing entity (will get paid personally by the insurance companies), then both of these NPI Numbers will belong to the subcontractor. If your subcontract is an employee and will get paid either by the doctors group or by the hiring provider, then the hiring entity's Type 2 NPI Number goes in Box 33a and the employee's (subcontractor's) Type 1 NPI Number goes in Box 24J - only if/when you bill Medicare and any other insurance carrier that requires the Type 1 NPI Number to be there. You don't need legal advice to figure this part out. This is information you already have to know in order to bill correctly.
If the subcontractor is an employee, then it is his employer's contracts with the individual insurance carriers that govern. If the subcontractor is the billing entity (expects to get paid directly from insurance carriers), then it is the subcontractor's contracts with the individual insurance carriers that govern.
Come back with any other questions you might have.