Returned IPhones for our company and locked out

Hi, I hope your well. We have several iPhones for our company that employees use. When they leave the job, quit , get terminated, or we buy them a new phone. They turn in their phones. Unfortunately we have several phones from former employees. That are locked and we don’t know the passcode to get into them. To re-issue For new employees. Are you at tech support or the apple store able to help me with unlocking the phones if I provide the account information or phone numbers, Or do I have to go to Verizon our service provider to unlock the phones?