I have been charged by O2 for some text messages I sent though I have unlimited free texts. When I asked O2 about it they said these messages were charged as multimedia messages because I included an emoticon in them. This never happened on my previous phone and I'm wondering what has changed. Apparently its not just O2, as there was a report on the news about 4% of users across all mobile phones being affected and EE was mentioned as well. There is some suggestion that it is down to how the handset treats emoticons. Does the Nokia Lumia (Windows 8.1) treat emoticons as pictures? If so, there is no warning of this that I can see.
Your network operator might charge for MMS messages (where you insert a picture, video clip, etc.), but they shouldn't charge you for SMS messages that use Unicode characters (and EE don't), which can include the emotji from the built in keyboard (via the 😊 icon on the keyboard).
Be careful to watch the character count, as this will force Unicode mode, which limits you to 70 characters per message (and this limit would be the same, irrespective of phone make or model). It might be the case that you've gone over the length for an SMS, and it's being sent as an MMS.
It may be worth following up with your network operator and challenging why you've been charged for those messages.
Additionally, if you've got Group text enabled, replies are sent as (chargeable) MMS messages, even if you don't include a photo/video clip/etc.