DWP

There is an option to Forward all calls, from a certain contact, to voice mail. That's good to have for annoying callers! When trying it I found that the caller is not forwarded to the voice mail but gets a busy signal. That can't really be the correct.
The same effect I found when using the Call filter to block out certain numbers.

What's behind these two options. Do they perhaps rely on using the phone's inbuilt setting to forward calls to Voice mail when busy?

Gorm

Hello
Yes, it depends on the system settings. Have you set up a voice mailbox number in the system.

Yes there is a number set for the mailbox. However, there is a phone function for Settings Call forwarding when Busy (Sony Xperia). That will do the trick, eg. a blocked user will be forwarded but that is not a proper solution as a call to a busy number should be met with a busy tone. A blocked number or a number that I do want to forward to the voice mail should not get a busy message.

Hello, I'm sorry in my country do not have voice mail service, I did not do a test on the voice speakers. But Android does not provide an API to control these.