Currently got an issue where my phone doesn't divert callers to Messagebank when I'm engaged.
If someone calls and I don't answer for whatever reason, they are diverted to Messagebank no problem. However, if they call and I am already engaged, they don't get diverted to Messagebank. They just get a single beep and nothing else.
You could try activating divert if busy, this is the code for Telstra, but should hopefully work too. I haven't actually tried it myself though.