Ok. Public information message for the older generation.
Make sure you place a security pin code on your Sim card. That way if someone steals your phone they cannot just take the Sim out, place it in their own phone and rack up bills even though your own phone May be secure (pin or fingerprint). It will be in the security settings of your phone.
Ensure your phone is locked with strong pin.
Android (probably similar for Apple) has security items for your Android devices. Log into google.com and select your account from top left icon. Select security. Scroll down until you find devices. There is a lost or stolen menu for each device.
Locate phone
Ring phone
Lock phone and place message on lock screen (lost phone call this)
Erase phone
Choose as appropriate