Using wi-fi isn't chargeable. Receiving texts isn't chargeable either.
You've not stated whether you put the credit on by simply paying for that credit, per se, or through buying a 'pack'. Packs can work out much cheaper (per call or text) for heavy users but they're time-limited (to either 7 days or 30 day, depending upon the pack chosen).
At the end of the relevant period all unused credit is lost. See page 2 here:
http://ee.co.uk/content/dam/ee-help/e-gain.s3.amazonaws.com/external/content/EE/Price%20plans%20and%20costs/P14_EE_PAYG_030914.pdf
EE also offers 'add-ons', which work in a similar way to 'packs' (i.e. they're time-limited). See page 4 here:
http://ee.co.uk/content/dam/ee-help/Help-PDFs/Business-PDFs/3GB%20PAYG%20Add-on%20pricing%20guide%20-%20amended.pdf
What I can't see in those tables is if your phone can charge you for data usage when you've not purchased a relevant pack or add-on. My own phone (on Vodafone), for example, has a symbol at the top of the screen that tells me the weather and temperature where I am. However (unless I turn data off) it uses a data connection to do so and, if I've not purchased a pack including data, charges me £1 per day for telling me what I could find out by looking out of my window! (So, as I only use my phone for calls & texts, I ensure that its data connection is always disabled).