I looked into this and the possible outcomes usually derive from being in a location with no service. But I'm assuming that may not be the case. I have some tools at my disposal that I can use to check out your device/ network functionality. If I can nail down that's it's a small glitch I can fix it.
Why do my callers (whose numbers are not blocked) hear this message sometimes when they call? "The person you are trying to reach is not accepting calls at this time" and then repeated in spanish, and then the call ends. They do not reach voicemail?