how to disable a lost phone

Have you set up Forgot My iPhone. See Set up your iPhone, iPad, or iPod touch https://support.apple.com/en-us/HT202033. If it is set up properly, including being turned on, see If your iPhone, iPad, or iPod touch is lost or stolen https://support.apple.com/en-us/HT201472. And, see this heading If Find My [device] is enabled on your missing device. See step 6. Don't make a mistake and remove the device from your account.