What is the fine-grained topic of the following text: Although most operators already offers 0 cents the minute, should know if they are limited to a maximum duration per call which would apply a price per minute higher than among other details that you know at the full rate per minute as the availability of 4G, if you have comparative cost to sail once exhausted the gigas or other alternatives with less megabytes. Only return the topic.
Understanding the details and costs of mobile phone operator services