O2 is a United Kingdom-based cellular network. O2 Simplicity is O2's SIM-only cellular plan. You can purchase the SIM card and insert it into any GSM phone. You can top up your account automatically, or buy a 12-month package. The cost varies depending on how many minutes you purchase. You can check your remaining balance and number of minutes through your phone or through the O2 website.
Send a text message containing the word "balance" to 21202, if you are on the Pay Monthly Simplicity plan, or 20202 if you are on the Pay & Go plan. You will receive a reply with your remaining balance.
Open your phone's WAP browser and type in the address "http://wap.o2.co.uk/myo2account" to log into your My O2 account on your phone.
Go to the O2 website on your computer and log into your My O2 account. You can find the link in Resources.
If you have an iPhone, you can download the O2 iPhone app from the Apple App store, or from iTunes. Search for "My O2" to find the application and install it on your iPhone.