Hmm.. I'm still not 100% sure I fully understood what you did, let's take it all step by step just to make sure shall we?
1. Go here and type *3* in the box, then press OK to save
2. Now create 2
brand new extensions leaving everything
default - do not change any of the settings so we can start with a clean slate!
3. Provision each extension to lets say, a mobile app or web client and call each other to make sure all is good. (A calls B)
4. Now let's modify extension B like so:

..which should create the busy scenario in all cases.
5. Finally call your extension A from any other extension, and answer. Now
blind transfer to B.
At this point B will send a busy, and A should start ringing again, having received the caller back.
Replicate this and ensure the system behaves as expected in this default scenario. Once validated, you can use extension B as your example to then go and compare your existing extensions.