Hey gang...
Will a IN9015-71 handset work on a 3515-71 (STS) phone ??

Even with the same shape, I don't think it uses the same transmitter so I don't think it will work. Doesn't hurt to try if you have one.

If I remember right, think I do, the IN9015 phone was a dealer specific phone for Infinity dealers and the the 3515 was the Graybar type distributor product.

You could ask Mike Sandman www.sandman.com or Refurb supplies https://www.refurbsupplies.com as they sell handsets.

I still have IN9015 phones and new circuit cards sitting in the warehouse that I would love to sell for most anything. Gave up being a Vodavi dealer and had too much stock.

Yes, the handsets between the 90XX and 35XX phones are electrically and nearly physically identical. The fit with the handset hanging up in a wall mount situation isn't perfect, but other than that tiny (and insignificant) issue, you will be fine.