Going somewhere that is not travelled often, does have the infrastructure to support uninterrupted service, or traveling on a mode of transportation that sells their access? These would be "premium".
A lot of providers will send you a text message to let you know that you've entered into these areas.
Most of these "premium" destinations won't be included in travel packages and you will have to pay a per minute, MB, or text (sending and receiving) charge to use the services.
For example, these are some of the charges I can expect if I use my phone on my next adventure:
CUBA
Internet access: 2G
Frequency: 900
Any calls: CAD $2.50/minute
Send text: CAD $0.75/text
Receive text: CAD $0.25/text
Data: CAD 1.50/MB with a minimum of 20 KB charge
Don't forget the taxes and service fess that may be added on top.
If you don't need to use your phone then put it on airplane mode so you don't get charged for receiving calls or texts.