I have an iPhone 4 (not even S) that works just fine as a phone.
I set up an old iPhone 3G (not GS) for my tech-challenged brother that works just fine as a phone --and as a music player, albeit with the giant rectal spasm that is (and always has been) iTunes.
There is no reason any iPhone will mysteriously stop working as a telephone because Apple no longer "supports" it.
If you have completely other plans for the use of your cellphone handset, or you just feel queasy because other people feel queasy, ignore this.
No, it is not like being an anti-vaxxer.