Mobile networks in Andaman are a huge concern for travelers visiting the Islands. Basically, the networks are extremely weak and 3G/4G are yet to become a trend in the island. We are at least 5 years behind when it comes to the network connectivity and internet in the islands when compared to mainland India.
Reliance Jio and Tata Docomo don’t work in the Andaman Islands. You will need to change your preferred carrier in case you are carrying sim cards from these networks. So the mobile networks that function at a basic level are mainly Airtel, Vodafone and BSNL. Among these nowadays it looks like Vodafone is providing the most clear signal. With Airtel, there is a lot call drop and with BSNL you would experience a lot of disturbance in the line.
If the weather conditions are perfect all the networks work pretty much the same, but in a case of bad weather, in most cases it is Vodafone that comes handy. I have had a personal experience with Vodafone and, I believe Vodafone stands out among other networks. I will rank BSNL as the second option and Airtel at the last for now. Though if you are carrying any of these networks to Andaman then you will be connected at least.
The second thing to know is while traveling to other islands BSNL might come as a savior as at some parts of the islands BSNL works better and in some Airtel or Vodafone. Around Radhanagar beach (Best Beach in India), Elephant beach at Havelock, Jolly Bouy at Port Blair no signal can be seen in your mobile irrespective of whichever operator you use.
Considering the remote locality of Andaman such things comes as a part and parcel of a tourist destination. Make sure your tour operator is aware as to where you will be visiting during the day so that in case of bad network connectivity we can send a person for assistance. At Experience Andamans, our representatives are at most of the places where such network connectivity issues are present and they coordinate/help all travelers who are in need of assistance. In addition, sometimes we provide an extra sim card to travelers who are facing issues with their network operator after arriving to the islands.