Why would a guy not respond after initiating a phone call?

So been speaking to a guy for a week we sending long messages 20 sentences+ we both know quite a bit about each other. He’s been very respectful.

He initiated the phone call after a few days of talking. We were meant to have one after I had work yesterday. However I have not heard of him. He ALWAYS replies to my messages no matter how busy he is.

Why would a guy not respond after initiating a phone call?
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