The call will go directly to voicemail without ringing if the phone is turned off or in airplane mode. If I call my phone when it's on DND from another phone, it does ring before going to VM.
I'd suggest not overthinking this. As far as I can tell, what you're seeing is normal. It is possible, though, that, sometimes, your boyfriend doesn't want to talk, maybe not to anyone, maybe not to you. If you're wise, you'll let him have that space. If he feels that you would be angry about that, yes, he might lie to avoid having an argument. Not the wisest strategy ever, I agree, but some people do silly things to keep the peace and/or avoid hurting people.