How Identify I phone nand storage capacity from on number

What numbers are you talking about?

From his phone number? Impossible. There's no connection between a phone number and device specs.

From his model number? Impossible. The 6s had three model numbers (A1633, A1688 and A1700) but those only tell you what carriers it will work with, not how much capacity it has.

From his part number? Yes. You would get that by going to Settings->General->About, but at that point you'd be able to see how much storage the phone has without looking up the part number online.

From his serial number? Yes, but you'll have the same issues as looking up the part number.

From his IMEI? Probably not. IMEI lookups will generally tell you the model number of the phone but not much else, or at least not much else accurately. I just did an IMEI check of my phone on several different sites. Most of them didn't list a capacity; those which did were wrong.