PF to be paid for exemployees of the companies

Hello,
We work with few companies on contract work. Deploying niche skilled contract consultants with our clients. We also don't hire such contract consultants. It is only a contract agreement which we sign with them, deploy them with our clients. Generally the duration of the contact is for 6 months or may be some times 7 or 8 months. We pay daily rate or monthly rate to them. One of our client is now asking us to pay PF for all our contract consultants. We are ok to pay to the contract consultants who is currently deployed with our clients now. But, our client is insisting us to pay PF for those who worked earlier and left the company long back. Is there any way or rule or act or any such shows that, we need to pay such consultants. The PF rule introduced by our client newly, it was never there earlier. Few of the consultants we are not able to reach them even. We don't know what to do.
Advise please.

do you have own EPF registration or not ? if yes then it was violation of EPF rules , for not paying the contribution of contract employees,
if you don't have EPF registration, then the eligibility criteria would be 20 for all employee including the contract employees, and EPF applicable on and from that very day on which the strength touched 20
if it never touched or crossed 20 in past , then simply issue an undertaking that your establishment has never crossed requisite manpower for epf mandatory registration (i.e 20) hence EPF can not be enforced on past employees.