Vietnam's GSM 900/1800 network is compatible with most of Asia, Europe and Australia, but not North America. In Hanoi you can buy a local SIM card and minutes for around VND100,000. The most popular providers are Vinaphone, Mobifone and Viettel. Until recently iPhones were only available on the black market; find them nearly everywhere in the capital city. For landlines, the city code for Hanoi is 04 (drop the 0 if including the country code) and the country code is +84.
When you buy a SIM card you'll get a local Hanoi number. You can send text messages worldwide for about VND3,000. A new, no-frills mobile phone can be had for as little as VND400,000, sometimes with credit included in the price. The store will usually set up your phone in your language choice. Take care when pulling out an expensive smart phone on the sidewalk, particularly along busy tourist streets in central Hanoi.
Mobifone / 26 Cua Dong st., Hoan Kiem Dist.; 043 7474 605
Viettel / 32 Phan Chu Trinh st., Hoan Kiem Dist. 046 2780 100
Vinaphone / 75A Dinh Tien Hoang st., Hoan Kiem Dist., 043 9363 601