Intercepting incoming calls on iPhone

Question

I would like to write software for my iPhone that intercepts incoming calls, and requires that the caller press "1" to proceed with the call. (This is for fighting telemarketers.)

I would prefer to write the software myself, and not install an app like Hiya.

I have a regular phone number assigned by my carrier (no fancy Google Voice or VOIP stuff), and I use the standard "Phone" app. My phone is not jail-broken.

What's the simplest way to intercept an incoming call to the standard "Phone" app?

(This is inspired by the Jolly Roger Telephone Company anti-telemarketer bot.)

Note: Some old threads (here, here) say it's impossible, but this thread mentions iOS 10's "CallKit" framework, but it seems it's geared more toward VOIP apps. So it seems like it might be possible.

Thanks.