The verb попросить is used differently in many word combination. You need "y" and the genetive case if you ask someone to give you something that he has and you (most probably) don't. This "y + gen" denotes the person you ask. You can mention the item you ask either before or after naming the person. Они попросили номер телефона у Саши will also be ok, but this sentence will tell you whom they asked to get the telephone number, while your example tells you what they asked Sasha to provide.

First at all, попрОсить :) Они попросили у Саши номер телефона They want Sasha's number, therefore they told Sasha to give phone number Я попросила об одолжении I want a some favor, therefore I told somebody to give me it Он попросил меня рассказать ей o правилах He want I talk her about rules therefore he talk me to do it Попросить is verb use when you want something and tell about it somebody and somebody can do it but shouldn't.