I would say US Cellular
Verizon currently has the best rated cell phone service in the US.
Att. Att has the fastest broadband network and covers 97% of all Americans.
You can view a cell phone directory at places like US Search, Spokeo, the National Cellular Directory, and Akiba. The National Cellular Directory would be your best bet. It is also free.
of course...
No, you do NOT have to purchase a replacement phone from US Cellular. I have included a link below to an online cell phone store that sells US Cellular cell phones. Hope this helps.
Talk to your phone-company they will do it for you for a small fee.
yes
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it depends on the phone you want to buy. your phone must be capatible with US Cellulars technology. This information can generally be found on the carriers website. US Cellular uses CDMA, so your phone needs to support CDMA. A GSM phone will not work on US Cellulars network.
No
Us Cellular is by far one of the best cell phone companies out there. They have amazing service and always are wonderfully helpful with any concerns you may have. They also are incredibly fast! My phone from US Cellular when I signed up arrived in two days. When I had a problem with my phone they Overnighted me one and the tech person was so nice he gave me his personal cell number in case it didn't arrive on time. When I ungraded my phone arrived in again two days and it was ordered Sunday night! They are number one in my book on services and phones.