What does it mean when a cell phone rings one time and then goes right to their voicemail?

Could mean that they do not answer unevaluated calls.

It may be what they set their answering machine to do. The first call is answered after so many rings, say, 4–8. The second and subsequent are set differently, say, 1–4. The first person you get a call from has to wait for the first set number of rings. The next callers only have to wait for the second set number of rings. So, if your call seems to go to answer quickly, chances are there has been a caller who left a message before yours. My answer confuses me. I hope you can make some sense of it. I am not particularly astute technologically.

It could mean that they’re on the other line and they hit the “decline” button.

It could mean that the call is being blocked.
On my landline (Comcast), when I set it to block a certain number, the caller gets the message “This number is not accepting calls.” They cannot leave a voicemail.
BUT, on my cell phone (AT&T), when I block a number, it goes straight to voicemail, but the caller can still leave a vm. I am not able to reject the call outright.

With my husband’s phone, four rings and voice mail, he didn’t answer. One ring and voice mail, he’s got no signal, and didn’t hear it ring. Zero rings, straight to voice mail, battery is dead. If he rejects a call, it still rings four times on my end.

I’m with @Judi I think it means they’re on a call.

I think it means the phone is off but I guess that depends on where the provider sets up the voice mail to go.