the traditional rules. The traditional rules state that you use shall to show what happens in the future only when I or we is the subject: I shall (not will) call you tomorrow. We shall (not will) be sure to keep in touch. Will, on the ...
the reality. The English and some sticklers about usage are probably the only people who follow these rules, and then not with perfect consistency. In America, people who try to adhere to them run the risk of sounding pretentious or haughty. Americans normally use will to express most of the senses reserved for shall in British usage. Americans use shall chiefly in first person invitations and questions that request an opinion or agreement, such as Shall we go?, and in certain fixed expressions, such as We shall overcome. In formal style, Americans use shall to express an explicit obligation, such as Applicants shall provide a proof of residence, though must or should works just as well here. In speech you can get the distinctions in meaning delineated in the traditional rules by putting stress on the auxiliary verb, as in I will leave tomorrow (“I intend to leave”). You can also choose another auxiliary verb, such as must or have to, that is less open to misinterpretation, or you can make your meaning clear by adding an adverb such as certainly.
§ 56. shall / will. 1. Grammar. The American Heritage Book of English Usage. 1996