To call within Vietnam you dial direct city code and phone number, to call within a city you don't need to dial city code.
Telephone Booth & Telephone Card: Using a telephone booth is an easy ( but terrible) way for you to call home. You can find telephone booths at post offices or in the street of major cities. Telephone cards are on sales at GPO, shops, restaurants, book stores. However, it is getting more popular and cheaper to make phone calls over the Internet. These days many Internet Cafes offer you this option.
International calls: Costs for direct dialed international calls are still high. However, you can make a phone call to talk with your relatives in your country with half of the cost with 178 or 171 services. With these services, cost is about 0.60USD per minute to most of countries in the world. How to dial it? Very easy:
Dial 171 (or 178) + 00 + country code + city code + number
The other option for calling home is buying a cheep calling card from VINA PHONE, FPT or VNN with the price from 30,000 - 50,000 - 100,000 up to 500,000. To call you scratch the card for secret code
Dial secret code + 00 country code + city code + number
Kindly note if you use this service from your hotel’s telephone, the charge might be a little higher as the hotel will put some service charges over it. Again, making international phone calls by Internet is another relatively cheap option.
Mobile phone: In Vietnam, GMS (Global Mobilephone System) is presently operated by three main suppliers: VINAPHONE, MOBIFONE, SFONE and Viettel. Your mobile phone could be used here by roaming service. These suppliers offer also VINA and MOBI Pre-Paid Card services. The best way for you to use a mobile phone in the country is to buy a pre-paid SIM card for your mobile phone. You can also rent a mobile phone at your hotel or at a Mobile Phone Service Center in the street.