They're thinking about what they want to say. You didn't see their voice message because you were looking for a text ( or vice versa). They are out of range. They are just really busy right now. You know...life. They just don't feel like talking right now. They'll tell you why later.
Rude or Mean Reasons You're Being Ignored. Because you want something they don't want to give you, like money or time. Because you keep messing up, and they are tired of bailing you out. Because they're afraid it's bad news. Because you're depressed and talking with you is a drag. Because you didn't get the job.
They just really don't care about your feelings. Because you're getting too clingy, desperate, demanding, or emotional. The quality of your communication isn't quite what they were hoping for and they have lost interest. They're feeling passive-aggressive and want to teach you a lesson.
They don't recognize your number and they don't want to respond to a potential stranger. They mistake you for a telemarketer. They're not in the mood to talk. They're a millennial, and millennials prefer texts to phone calls.
Good Excuses for Not Calling Back. They're at a cellphone-free place where it's not okay to answer calls or pull out their phone, like at work, a library, or a movie theater. They're driving (it is against the law to use a phone while driving, after all). They're injured or really, really sick.