It could be a couple of things. They could either have no signal or they could be ignoring the call.
Sometimes if they have no signal it will take a couple of rings for the phone to realize that. Then only AFTER the first couple of rings will it send it to voicemail.
Or, the person could also be hitting the "ignore" button.
Most likely tho it's that they don't have any signal. If this is the case, try sending a text message. Sometimes if the person's phone doesn't have enough signal to receive a phone call it can get enough signal to receive a text real quick.