Among the handful of things that might automatically send your calls to voicemail messages is your phone's Do Not Disturb mode, blocked numbers, call forwarding, Bluetooth connections, and your phone's SIM card. Whether you accidentally activated any of these phone features or forgot you turned them on, don't fret.
Have a look in his call settings and see if there is a block list, scroll through his contacts and see if there are any sort of marks against any numbers.
I have a Wiley Fox swift 2 android phone. Husband has a STK android phone. The problem is that whenever I ring his phone, it goes straight to voicemail. He can ring me! He receives calls from everyone else, what's going on? Any ideas gratefully received. Thank you.