why do calls go straight to voicemail on my iphone

DND off isn't the whole story. Check Settings > Focus for any schedule, a Sleep or Work focus turned on by time or by your Watch silences calls the same way. Then Settings > Phone > Silence Unknown Callers, that sends anyone not in contacts straight to voicemail. Also Settings > Phone > Call Forwarding to make sure nothing's set. If all clear, it's the carrier, ask them to reset conditional forwarding on the line, it gets stuck after number changes. Full bars doesn't mean the network is registering you for voice.