My mobile phone keeps receiving the text message "This number is using the mobile phone". Why?

In this case, I don't know the situation, and then I set the settings such as anti-theft tracking on his mobile phone, and the mobile phone will automatically send a text message to your mobile phone, and your father will install his own SIM card on his mobile phone.

According to the analysis you described, either cancel it.

Just reset the anti-theft tracking settings on your dad's mobile phone? Either you used to install the SIM card on your father's mobile phone, or your father used your old mobile phone.