A strange number sent a message saying that the courier had been received. What happened?

I should ask your courier's personal phone number, because I am a courier and use the number issued by the company to go to work. Some recipients can't be reached by phone or are not at home, so we will take them back to the company and deliver them another day, or leave them at his neighbor's house or in the shop downstairs. It suddenly occurred to me after work and I asked with my own number.