It is my firm belief that you cannot change what blocked callers hear when they call you. But that’s an awesome idea! I would love it if you could assign a song to a blocked caller and it played for them instead of hearing the ringing, and when the song is over, screw letting them leave a voicemail. The phone should automatically hang up on them after the song is over.
Here’s a perfect song for a blocked caller.
Or this one (NSFW):
http://bit.ly/2f8aiPk (This is a YouTube link, but I couldn’t post the actual link because NSFW language would have shown in the still image [thumbnail].)
I believe I just told you that it is my firm belief that what you want isn’t possible. Please click on the link I provided. It describes exactly what happens when a blocked caller calls you, and the sequence of events described there is immutable.
But on the off chance that I’m wrong, but I’m going to test your “busy signal” claim and I will get back to this thread when my test is complete.
I apologize in advance if I’m wrong.
Maybe someone in the j/b crowd here on eiC can offer a j/b solution. If so, I’m sure they’ll chime in soon enough.
Update: I just asked my friend to block me temporarily. I callled him. What happened was exactly what the article I shared earlier describes. So, to summarize: No. What you want is not possible, at least not on stock (non-jailbroken) iOS 10.
Look at YouMail for your voice mail app. I've used it for ages and really like it. Custom messages and I do believe you can select what you want a blocked number to hear.
App Store link:
YouMail: Visual Voicemail Replacement by YouMail, Inc. https://appsto.re/us/eqmys.i