Is call recording not possible in iPhone 6S for free of cost.

Call Recording

Federal law (In majority of the Countries) requires that at least one party taking part in the call must be notified of the recording. 

For example, it would be illegal to record, without notification, the phone calls of people who come into a place of business and ask to use the telephone.

If your country region allows call recording, you may find third party app for call recording. Paid and Free both. Features may differ.