First, read the English article below.

Some SIM cards do not display their number inside the settings of any phone. For these cases, you may have to call the service carrier of your SIM card and ask. If you are calling from a phone with the mystery SIM card, the carrier may be able to detect the number automatically. If not, have the SIM card at hand so you can read aloud the serial number. Use the SIM card whose number you want to know. Any phone with caller ID will detect the number of your SIM card, unless you are using a private number. 

Now, please write a short abstract for it in Vietnamese. Abstract:
Gọi cho tổng đài. Gửi tin nhắn hoặc gọi cho một số khác.