a pending case against me.

...if there was a pending case against you, you wouldn't find out about it via a threatening phone call.  Block the number.  This number is also a Google VoIP phone number.

Daniel Matthews left a voice mail on my cell phone regarding my ex-mother in law from over 16 years ago. He called from 951-387-5063 but gave 626-470-7491 as the number to contact. They had better find another route to find her besides calling my cell phone. If they think I'm going to track an ex down from 16 years ago for them, they have lost their minds. I'm getting several no caller ID calls and so now I'm guessing it's her bill collectors. I shouldn't have to change my number to stop being harassed.

There's a very good possibility that it is a extortion scam operation.  And no you shouldn't have to change your number. For a cell phone, some already have call blocking, if not, there are free apps on the web that will work for some. You can google this. If you have a smart phone, there is a free app called call control. Iphones appear to have apps also. Another one is called Mr. Number.