Just like other European countries, the cell phone penetration is very high and most inhabitants own a mobile phone.
This said, it is good to know there's on only one phone operator in Andorra: Andorra Telecom.
Due to its small size and extremely mountainous landscapes, Andorra's one and only operator is the state-owned Andorra Telecom. You can get 2G, 3G and 4G prepaid packages if you have an unblocked GSM phone.
Once you are in a good coverage area, you will be able to pick up a strong signal.
Notice that Andorra is not part of the EU, so it doesn't have to abide by EU roaming laws.
To get a mobile contract you will need a passport or your photo ID for proof of purchase.
Most of the contracts can be easily terminated or changed on a monthly basis. An exception are contracts that come with a phone, in which case, a contract cannot be terminated earlier than within at least 6 months/1 year, depending on the service terms and conditions.
See that if do not know how much time you are going to spend in Andorra and you prefer not to be tied to a pone contract, "Mobiland Clic!" pre-paid card allows you to make calls, send SMS messages and browse the Internet without any monthly fees or invoices. So you can decide how much you want to spend.
In any case, there are some basic points you need to consider for choosing the right deal/package:
See that Andorra is not part neither of the European Union and the European Economic Area (EEA). Consequently, roaming will be charged on international calls, texts, and data.
This said, mobile phone customers will be charged international price for calls, texts and data when travelling to Andorra. We recommend you to check with your service provider to confirm your data allowance before travelling.
You can read more about the "Agreement to end roaming charges from June 2017" here.