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. >_<

That means " telephone so much use as well as basic charge for telephone company is also expensive. / This common excuse for not use telephone not used now. amari tukawanai = not use often Kihon ryoukin= Basic charge ( even not use charge by teephone company) takai = in this case meaning expensive.