I'm studying Japanese and we went through this grammar last year. This is what we learned in class, so I hope it will be useful to you.
"amari" means "not much" or "not very" and it's always used with a negative form of the verb.
"shi" is used to talk about causes or reasons and it's like saying "~and ~ and ~and..." For example, if someone asked you why you don't call them often, then you could answer with the sentence you wrote above, which would be like giving excuses or several reasons for why you don't call them. "I don't use the telephone much and the basic tariff is expensive." Using "shi" would mean that there might be more reasons, it has this connotation of "and so on". So even if you use it only once, it's as if you are leaving other reasons unsaid.
I think that there are other uses of "shi" but I don't know if I can explain them well. >_<