Ee Time Scale For Cancelling Phone

[Handoftheking]

I have an account with ee for my mobile . I ordered a new iphone 7 with 128Gb memory. I need a lot because I am about to spend some significant time in hospital and use the phone to watch downloaded TV programmes and to listen to music. My previous phone had 32Gb and it keeps getting full. I have a lot of photos too.
My son negotiated a reasonable deal for the 128Gb version, but they sent the 32Gb one. He has been trying to resolve the situation but they are really stringing us along. They promise to ring us back and don't. I've more or less resolved to kick it into touch with ee. (Not keen on Kevin Bacon anyway :) My only worry is that I will have gone beyond the time for being able to cancel. the invoice says the 16th Feb. I may go with O2 even though I've been with ee and T Mobile (before they were swallowed up by ee) for years. I have tried to find some info on the web site without success. I cant ring up because I have no voice due to radiotherapy.
Any help or advice appreciated