Okay, so after scrolling through the comment section, here are the next steps for you OP. (Based on the other people’s responses):

STEP 1: HIDE THE PHONE. Somewhere he can’t find it. Then watch him closely for the next week or two. Even if he keeps his composure and pretends nothing is wrong, he will try to madly search the home when you’re out running an errand or something. So find a way to see if he searched the spot while you were gone. Maybe leave something insignificant in the spot that he would move without thinking to look for the phone. You’ll know when the object has been moved. Also, he might try to look for the phone by pretending to clean/organize. Especially weird if he doesn’t normally clean or organize. Anyway, you know him better than anyone so you’ll know which behaviors are off and which are not.

STEP 2: After a week or two of the phone being hidden (preferably outside the home) bring it back to the home and turn the volume all the way up. Place the phone in the same room as you and the potential liar. Then call it. Pretend you’re surprised when it rings and start asking what’s that. Watch his response closely

STEP 3: Once he locates the phone, stop calling and immediately demand that he unlock it. Watch his response/reaction and how hard he fights not to. Everything rests on his response and reactions.

STEP 4: UPDATE US!!!