I increased my limit without having to give a mobile phone number, which I don't have anyway, I just clicked it and bam it increased. Also, even though I created my latest channel in the past few months it seems to be in the style of the one the older forms of youtube, and not the newer ones, so that may be why I didn't have to give a number. I don't know why it turned out that way but I prefure the older school version of YouTube anyway so I'm happy with it.
To answer your question so far as I know once you're past the 15 minute limit I think it's unlimited. The longest videos I post are about 23 minutes.