Summarize:

This can be one of the first signs that a guy is losing interest. Usually, you can safely assume that a guy will contact you if they want to talk to you. When a guy that usually calls and texts you frequently stops doing so, you should take note.  More isolation and less verbal contact can be a sign of declining interest. But, it might also signal changes in his personal life – maybe he’s very busy or has issues to deal with at home. It’s always best to reach out and ask directly rather than just assume he’s lost interest. This goes hand in hand with reduced communication. If you start to notice that when you do talk, it’s only for very short periods of time you should be aware of that.  For example, if you go from talking on the phone for hours at a time to only speaking for ten minutes at a time, this can be a sign that he’s losing interest. He may try to shorten your conversations by ignoring your texts or calls for long periods of time before returning them, or saying that he has to stop talking but doesn't offer a clear reason why. This is a strong tell that he's not interested anymore. Many men will speak to people that they’re interested in with a softer and more flirtatious tone than they would use with others. When his feelings change, his tone of voice becomes less affectionate and starts to sound like his normal tone. He may even speak unkindly towards you, or say unfriendly things. This is a clear sign that he is not interested in you. You do not deserve to be around someone who treats you poorly or says harsh or unfriendly things to you. If he ever starts to say disparaging things about your looks, the things you do, or how you act, you should end the conversation immediately. When you two are talking, does he ask you about meaningful things in your life or does he focus on himself? A guy that is interested in you will try to have deeper conversations and try to get to know the real you. When he speaks, notice when he tries to keep the conversation superficial or tries to cut it short.  He may be trying to subtly hint to you whether he is interested or not. You should pay close attention to the things that he saying, as well as the way that he is saying things to you. Does he joke with you as much? A lack of humor might mean he’s feeling low – maybe depressed – or just a loss of interest in general. Also note whether he talks about trivial things or goes into deeper subjects. Avoiding topics of substance or talking about his feelings may mean he’s not interested. If he starts to talk about his relationships with other people, especially in a romantic way, he is letting you know that he sees you only as a friend.
Look for reduced communication. Watch out for shorter conversations. Notice when his tone of voice towards you changes. Listen to what he says.