Here for advice on a stolen iPhone

Long story short: phone purse and car were stolen last week in Missouri. Reports made and all that, including that it was stolen to Verizon. Checked my voicemail today and someone answered. Verizon claims to have no record of the call, but it’s dead now. The phone was marked as lost by us, and my wife tracked it and could see that it had been powered off as soon as they took it. It appears to have been turned on and accessed somehow today, with lots of calls and texts made. I’m just trying to figure out how it was accessed and depending upon that, what was exposed. We’ve changed every password we can think of at this point. Are there services that can help to investigate the extent of the exposure or help monitor? Needless to say, any insights would be greatly appreciated.