How to use mobile phones conveniently when traveling in Vietnam?

It is more cost-effective to buy a mobile phone card in the local area, and it is not expensive to make a phone call there. If you use your own mobile phone, it seems to cost several dollars to send a text message. It seems that you have to pay 500 yuan for your mobile phone first, and bring it with you when you travel to let others find you. You must have contacted someone else. No, contact whoever you want, and don't contact whoever you don't want.

Opening related international business in the business hall:

1. Pay a deposit of 2000 yuan to Mobile, and Mobile will help you to open international telephone service, that is, make international long-distance (Vietnam) calls in China or make calls in Vietnam. If the mobile phone has a signal, you can make calls and receive short messages.

2. Pay 1000 as deposit, but only for making international calls at home. (That's what I do)

However, I don't know why. When you go to Vietnam, your mobile phone also has a signal, so you can make a phone call or send a text message. My mobile friend doesn't know what's going on, saying that it will only happen if I pay 2000 yuan ~