How to tell if someone blocked your number

My friend has iPhone and his carrier is either Boost Mobile or AT&T and this doesn't happen every time I try calling him but occasionally when I call him the call rings once then goes to voicemail other times it rings the normal 5 times before going to voicemail. Have I been blocked or are we having technical issues or could his cell service be disconnected due to non payment? I don't have any other way of getting in contact with him besides calling/texting him.

On a side note, we have had issues receiving texts from each other before. Every single time he has sent me a picture, I will either get the pic right away but it will show in the previous days texts in a random place or it takes a couple days for me to receive it.