This list shows these countries where it's impossible to buy a local prepaid SIM card (for data) at the moment. This can be because of lack of technical infrastructure or legal reasons which prohibits the purchase of a SIM card for foreigners. As you see, this list is rather short:
In Cuba there are still no data on their GSM network, not even GPRS on the only state-owned mobile provider "Cubacel". The only way to log on the internet for foreigners is to go to one of the major hotels or public WiFi hotspots. The situation is about to change with the easing of the US embargo. First 3G cells are now operating in Havanna and Varadero, but for roaming customers only.
Eritrea was the last country in the world to adopt a mobile phone system. It's sole provider Eritel is hardly accessible to foreigners as it requires a resident's permit, a very high connection fee and some weeks to get connected to a network mostly 2G-only with few 3G cells. There is a separate CDMA network covering 85% which is not compatible with most GSM phones.
The state-owned provider "koryolink" sells SIM cards to foreign visitors for their 3G network on 2100 MHz: a "Visitor Line" with 3 different packages on prepaid and a postpaid "Foreigner Line" for residents. Unfortunately, the Visitor Line has voice and text only, but no data. This can only be booked on the Foreigner Line at astronomical rates, but is inaccessible to short-time visitors.
The only provider Telecom Niue has a GSM network, but not for data, only voice and SMS. Visitors can have internet connection through Niue's extensive WiFi network with hotspots all over the island after paying NZ$ 25 for 5 GB in 14 days (for more info check here).
Transnistria is the only populated region in Europe without a 2G or 3G GSM coverage as the only provider "IDC" uses CDMA and EVDO only which is incompatible with GSM-devices. This situation may change as a 4G/LTE network on 800 MHz is being built, but not yet open for prepaid.
Tristan da Cunha, Pitcairn Islands and Tokelau Edit
In those territories there is no mobile coverage whatsoever. Your only options might be an internet café (if available), landline internet including WiFi (if available) or satellite internet.
Greylist EditThis list shows these countries, where you should think twice of buying a local prepaid SIM card, as it may not be advisable under all circumstances. Because of procedures, legal barriers or technical restraints, you may be better off looking for alternatives like WiFi or an international roaming SIM. But check articles first to see whats restricted and if it applies to you and your purposes.
To buy a SIM card in Algeria, you need to present a certified copy of your passport to the provider. This needs to be made at a local mairie (town hall) at office hours before you walk into any operator store.
It's easy to buy a local SIM card from one of the providers as a new centralized registration system for foreigners has been implemented. The country remains greylisted as an additional IMEI registration system is in place for all devices using a local SIM.
Because of the "Great Firewall of China" a.k.a. censorship your internet can be severely restricted. Facebook, Twitter, Google sites like Maps, Gmail or its search engine and many others are blocked. Furthermore, China is starting to clamp down on VPNs used to circumvent these blocks.
Diego Garcia Edit
This remote island in the Indian Ocean is served by Sure on 2G only. There is landline DSL at low speeds too, Lacking a submarine fiber link, prices remain high and speeds low for C-Band satellite connections.
Equatorial Guinea Edit
Foreigners can buy a prepaid SIM card not in usual operator's stores, but only in two special branches of GETESA (Orange) in Malabo and Bata.
Falkland Islands Edit
The only operator Sure on the islands has 2G only up to EDGE speed without 3G or 4G, but almost everyone in Stanley has ADSL internet service instead.
India has a very unforeseeable policy of registration which can take a few hours but also more than a week. So you'll never know, when you will actually be connected to the network after having purchased a new SIM card.
Japan is one of the few countries with no 2G/GSM network and much of its 3G is on very own frequencies. Furthermore, it bans the purchase of voice SIM cards to travelers by law. But you can still buy data SIMs or rent a device or SIM card at many rental agencies.
The Palestine Territories have their own networks, but are restricted to 2G and data up to EDGE speed only. This is due to the refusal of Israel to grant 3G spectrum to Palestine operators. You may use Israeli providers instead.
Saint Pierre & Miquelon Edit
The two providers SPM Télécom and Globatel only offer 2G up to EDGE speed in this French Overseas Department. This leaves this area as only territory without own 3G coverage in America, but wired internet access and a widespread public WiFi network are readily available. A 4G coverage is planned.
Turkey has implemented a scheme of paying a TL115 tax for all imported devices to be used with a local SIM card. This is added by a whitelist registration system of IMEI numbers. If you don't follow the rules, your device will be blocked after a few months. So it only bothers you, if you want to stay longer or keep your SIM for another visit.