Bhutan

The Kingdom of Bhutan has two mobile network operators: 2G/GSM is on 900 MHz on both providers and 1800 MHz on TashiCell, 3G up to HSPA+ on 850 and 2100 MHz in the major towns only. B-Mobile has started 4G/LTE on 1800 MHz in Thimpu only. Be aware that data speeds can be painfully slow outside of the capital.
 * B-Mobile (by Bhutan Telecom, state-owned)
 * TashiCell (privately owned)

Regulations
Although Bhutan remains relatively closed to international tourism only allowing in a contingency, as a visitor you are free to buy a prepaid SIM card. Officially, you need to give them a passport copy and an additional photo of you. In their airport offices, they are able to do the copy work for themselves. Additionally, you have to fill out a form like this (in PDF). Airport offices might be a bit swifter with the paperwork, but you can go to any office of the operators.

B-Mobile (by Bhutan Telecom)
State-owned incumbent Bhutan Telecom Limited (BTL) is the leading provider of telecommunications and internet services in the Kingdom. Besides fixed line telephony and internet services, it provides mobile services under its brand B-Mobile. It has the larger network with more users.

In 2013 it started 4G/LTE in .....

As soon as you arrive in Paro internatl. airport in Bhutan you can avail a prepaid SIM card by just giving them your passport number and filling up a simple form. After completing immigration procedures, you'll see Bhutan Telecom counter right besides the money exchange counters.

Most of the time they don't have any Micro/Nano SIM, so if your phone doesn't take regular size SIM card, you have to cut it.

The SIM comes with Nu.100.00 preloaded, so you have to pay that upfront.