Verizon wireless contract plan to start credit?

I'm 20 yrs old and trying to build credit. When I was 18 I got a credit card, basically maxed it out and then payed it off but let the balance sit for a while but less then three months or so. My credit score from Experian said I'm at no credit so I recently got got qualified for a plan with Verizon so I went ahead and got an iPhone. Going to be paying about $100 dollars a month for two years unless I pay off my phone sooner which will reduce my monthly deal to about $65. My question is, how effective is this? Main reason I did it was so boost my credit. If this isn't an ideal way to do so, what are some better ways?

Not effective at all - these plans are not considered credit and the phone companies do not report to the credit bureaus (unless of course you don't pay and they turn it over to a collection agency).

Get a credit card

Use it

Pay on time

Not effective as this is not regarded as "credit".

You don't need credit (FICO score) until you want to buy a house or investment property. Start by saving to buy a car for cash. Look for a private sale for $1-2,000. Take it to a good mechanic before you hand over the cash.
Pay your rent on time every month and be a good tenant, no loud parties, etc. Pay Verizon and all your utilities on time. Letters of reference from these companies plus a verification from your bank about your account should get you a mortgage from your own bank which can manually underwrite it.
Use your bank's Bill Pay system to help you pay on time. Try hard to save 1/4 of your paycheck every week. Once you do it will be easy to buy things you need.