Summarize this article in one sentence.
Some carriers will let people with no or poor credit sign up for a plan, but you must pay a security deposit first. For example, T-Mobile lets consumers with the best credit buy an iPhone for around $27 a month. However, if you have poor credit, you can pay $360 upfront and then make monthly payments.  To estimate your security deposit, use an online security deposit calculator. You need to know your credit score to use the calculator. You can get your score from myfico.com for a small fee, or you can use a free online service such as Credit Karma. Stop into any store and apply, or apply online at the carrier's website. You typically must provide the following information when you apply.  personal identification, such as a driver's license or other government-issued ID telephone number email address delivery address tax identification number The carrier will check your credit and then contact you about a security deposit. Typically, you must pay your deposit before the carrier will activate your plan. Some carriers won't even let you pay a security deposit. Instead, carriers have created prepaid plans for people with bad credit, so don't be surprised if you are rejected. Your carrier will probably refund your deposit if you pay your bills consistently for a year. They should pay simple interest on the deposit, so you'll get back a little more than you paid in. Make sure to pay your bill in full and on-time.

Summary:
Calculate your security deposit. Apply for a plan. Pay your security deposit. Receive a refund of your security deposit.