Just to recap, dont go rushing off with the idea that there is a deal that gives everything, network providers give the impression that they can supply such a thing but you need to read carefully and more importantly read between the lines. I know I keep saying it but there is no such thing as a free lunch.
No network provider has enough bandwidth to cope with unlimited data all of the time, you can choose either unlimited data on a phone sim which, some of the time, is restricted or you can have a limited amount of data on a data only sim (not to be used in a phone) which is for a limited time but has the advantage of taking priority over a phone sim in busy times. That's why unused data doesn't roll over to the next month otherwise they would be snowed under trying to supply promised data over an already crowded network.
Network providers have to do a difficult juggling act to keep most customers happy most of the time even if that sometimes involves signing them up in the full knowledge they won't be getting what they think they will. Things can easily be papered over when verbally agreeing, especially when some young staff members aren't sure of the setup themselves.