I don't understand why the deposit should be anything over $175 (the cost to cancel the contract). I figure that it should be $175, and if you are too late at paying your bill, they cancel your contract and keep the $175.
Deposits are high in case you go over your minutes and rack up several hundred dollars a month in standard and extra service/overages. They will let you not pay your bill for several months (3-4) before they terminate your line, and if your bill is $200/month with service and overages, that can easily get up to $600-$800. However, they figure people with good credit won't be irresponsible like that and will pay their bills on time. Also, the prior poster was correct that cell phones and utilities don't build your credit profile. They are dependent on your credit, but don't contribute anything to it. The only things that build your credit are, of course, loans and credit cards (but only if you pay them on time!)