The Subscriber Identity Module, or SIM card, is an essential piece of equipment in your phone. Your phone will have a broadband chip which allows it to use a cellular signal to communicate information from your device to another, but without a telephone company's SIM card, your phone will not be able to access the provider's cellular network. While the SIM card gives your phone the ability to access mobile networks, it also acts as a leash to limit your options.
I'm a travelling sales representative, and I spend a lot of time on the phone. I always used to be looking for a charger and it was a total pain. I switched to a phone model with much better battery life, and I have a power bank that I travel with that allows me to recharge on the go. Often it can make a lot of difference when you switch off apps and adjust phone features such as brightness. This blog has lots of details on what you can do to improve battery life, and it also discusses which telephone models have the best battery life.