Keep phone from ringing

Do Not Disturb might do what you're looking for. Not sure about calls going to voicemail, says callers will get a busy.

Turn the ringer off.

If you just don't want the phone to ring do as suggested by @skeeterintexas . If you actually want to block all calls, which makes no sense to me, you'd need a 3rd party call blocker, can't be done through the phone company's blocking service. 

Or unplug the phone.

I have had a code from ATT that gives callers a busy signal. The phone is still usable from my side. I did turn off the ringer this time but fills up my voicemail.  Looking for the code. 

There is no such code that I'm aware of 

STAR CODES

I had it and it worked for a year. Somehow I accidentally disconnected it. Now I can’t find the star code that activates it again. 

I have the ORIGINAL “Features Guide” provided in 2008. 

Perhaps they added some sort of 'ringer off' code later.  I use *61 to block the last incoming call (I use it all the time) and it's not listed here.

Perfect. That’s it.