For your first example, it’s a regular use of the imparfait: it means you were doing something when something happened.

I was at the cinema can be understood as I was being at the cinema, right?

It was the situation when something happened (the phone call).

Your second example has zero context as I’m writing this, I’ll edit if you precise your example.

The imparfait is for action which are not completed yet at a moment of reference which should be clear from the context. The action itself can be a once off action or a usual action (in which case the action can in fact not be occurring at the precise time, it just had to be habitual in that time frame). The moment of reference can be determined in a lot of way including another action (usually expressed in the passé simple or the passé composé) or just be implicit. It is quite common to have a description of several phrases in the imparfait and then have the point of reference being the main action in the passé simple or the passé composé.

There is also a bunch of other usages more or less related (after si, children are using it to set up the context of their play, …)

You first example is typical: the moment of reference is the call and you were still at the cinema at that time.

The second example does not provide enough context to choose how to translate I had two slices of cake. With the imparfait you are giving the impression that you are setting up a scene for whatever story you want to tell. With the passé composé you are giving a pretty uninteresting random fact about your past. Considering the meaning, the imparfait seems a little more probable, but the choice is not driven by grammar consideration but by a guess about the context.

Edit: your second example after edition:

I had two slices of cake : J’avais deux tranches de gateau parce que j’adore gateau.

I’d not use the imparfait here, you are not considering a uncompleted action, with the imparfait I picture you holding two slices of cake, one in each hand. And the verb to use here is not avoir but prendre. J’ai pris deux tranches de gâteau parce que j’adore les gâteaux. (It’s not a case where we’d use a noun without an article in French).