Go with T-Mobile
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I have recently signed up with Cingular cell phone service but I don't really like their service. They advertise saying they have the least drop calls but I've experience at least two times of drop calls within the first month of joining Cingular. The rebate filing was a hassle too. I had to call their customer service several times regarding my rebates before they approve my rebate. Such a hassle.
I wouldn't mind switching back to T-Mobile. For the past 3 years I've been with them, there was not once a drop call. Their customer service is very efficient either and their coverage in Los Angeles area is very good. I didn't have any problems with my T-Mobile cell phones either. In fact, I prefer my Nokia cell phone than my current Sony Ericsson. I think Nokia phone is more user friendly than others.
If you are looking to sign up a new cell phone service plan, GO WITH T-Mobile. But remember to visit Wirefly.com as they probably can offer you better deals than most brick and mortar stores. Some of the deals even come with free accessories (free bluetooth headset) and rebate offers that may bring down the price of your cellphone even lower.