Why are callers being prompted for code when calling me?

I do not use a smartphone, but my AT&T cell phone does have internet capability.  I do use texting, but not the internet.  When I am visiting a somewhat rural area, local people are unable to reach me on my phone (but my partner can reach me every time, from anywhere).  The neighbor's calls go straight to my voicemail and my phone never rings. I don't actually receive their voicemail notification until hours later.  In another case, I called a library after hours and left a message to sign up for an event.  I called again later when they were open and they said that when they had tried to return my call, they were prompted to put in some sort of code number, and of course had no idea what to do.  I don't have any passcodes for people.  Funny that marketing scammers can reach me, but legit callers are asked to provide a code.  Why is this happening?  Is there anything I can do?