As for my carrier I was on tmobile, for 6 years, untill I got fed up,.. I paid $10 for an online company to send me an unlock code for my Android phone its a sensation 4G,
..now I can jump from one gsm prepaid carrier to the next whenever I want depending on who has the best service where I will be camping
...... Gsm carriers are basicly ATT and T-Mobile, they lease their tower to lots of prepaid carriers. Straight Talk, simple mobile, go phones, ect
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. Any unlocked tmo or att phone will work on the others network but unless it's Pentaband it will only have the ability to use the other carriers 2G network.
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Lots of newer phones from both companies are Pentaband but I recommend the galaxy nexus (unlocked $349 from goole play store) or the new LG nexus coming in the next week price yet un-announced.
Being unlocked and direct from Google are a huge plus aside from being Pentaband, and nexus phones (nexus phones are pure android no 3rd party bloatwear so they always get the newest software updates first)
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Straight Talk has 2 kinds of Sim cards one for att network and one for tmobile, when in for example lone pine ca I use at&t straight Talk Sim because tmobile has no coverage at all, in lake Isabella both have strong signal but tmo only has 2G while at&t has 3G,,,,,,, however if I'm in bakersfield I use a tmobile Sim for their kick a$$ 4G.
Most prepaid plans cost $40-50 a month for unlimited everything and straight Talk they will throttle to 2G though after using 2gb a month, (best plan for more 4G before being throttled is tmobiles prepaid $70 a month unlimited everything 5gb before throttle)
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Currently I'm using a straight Talk Sim, but soon I'm going to take it up a step.
I tmobile also offers an online sign up only plan 5gb before throttle unlimited text and only 100min talk, and for $30 a month,,,,,,,,,, then for more talk time I'm getting a 2nd phone through the lifeline cellular program aka obamaphone,,,,,,, for $20 a month they will give me a dum-phone with 1000min. That uses the Verizon network, so $50 bucks a month and I will have plenty of talk, text, and data AND access to all 3 of the biggest carriers networks for the best possible chance of a signal where ever I'm camped