AT&T and Unlock cell phone

故事是這樣的，本來我的手機需求就不大，所以一直用 3.5G 手機。所以來美國後，就用一樣的手機，申請了 AT&T SIM 卡就用了。反正網路也沒問題。但是隨著手機電子零件的耗損、電池的老化，GPS 定位速度越來越慢，還有 App 越來越肥，還有常常有些閃退 (手機是 Atom x86 系）。所以又換了一隻新手機，雖然舊手機用起來還很快，RAM 也是很大夠用，但是剛好用到出槌點時就會罵一下。
現在手機幾乎是很難買到沒有 LTE，而且是 Unlock 也相容 AT&T 4G 的頻道。但是換上 SIM 卡後，還是 H+ 圖案沒有 4G。但是使用 Android Setting 裡面的 Search Network 可以看到 AT&T 4G, AT&T 3G, AT&T 2G。選了 AT&T 4G 但是會一直沒有訊號。
後來找找網路的論壇，大概是 AT&T 會用 IMEI 去鎖 mobile network。不是他認可的 brand/model IMEI 是不能用 LTE。所以解決辦法就是，打電話給技術客服，請他把你的 IMEI 設定成可以使用 4G。解決辦法有兩個:
1. 去借一隻 AT&T 手機或是用他們認可的機種，先把 SIM 卡放上去，打客服說你的新手機是這個 IMEI，因為特定 brand/model IMEI 在 AT&T 就是預設可以通 LTE，這樣就解決了。AT&T 網頁有檢查 IMEI 是不是他們認可的支援 LTE 手機。
2. 有的技術客服比較 nice 的，他就直接幫你在他們系統弄一隻 dummy 可以通 LTE 的 IMEI，反正只是給系統檢查用，不影響平常通話。
因為他們系統似乎是寫死的，只能靠帳戶 IMEI 去認能不能用 4G data plan，而不是靠 SIM 卡上的實際使用 IMEI。所以無論如何只能透過上面兩種方法，目前無法用他們系統不認識的 IMEI 去用 LTE。大概也是因為這樣設計，所以才能用大家買他們的手機綁約。
等客服弄完之後，把 APN 設定對，然後開啟飛航模式再關掉，讓手機重抓網路。就可以在上面看到 4G 的圖案。但以後換手機或怎樣絕對不要請客服去更新 IMEI，不然可能又會換回無法識別的 IMEI 就無法使用 4G。
大功告成收工，運氣不好的打到某些客服不懂得這是啥問題的，講不清楚就再換時間打換不同客服試試。

I bought a new 4G phone. After I moved my SIM from the old 3.5G phone to 4G phone, my new phone still shows 3/3.5G H+ icon, not 4G. I googled and found some similar discussion on forums. The reason why I cannot get 4G coverage is AT&T uses your registered IMEI number to identify your phone is 4G or not. If your IMEI is not their recognized 4G phone, your data plan will be up to 3.5G only. So there are two methods to resolve this issue.
1. To borrow a AT&T mobile phone or get the same brand/model phone. Put your SIM on that phone and call AT&T customer service representative to update your registered IMEI. The technician will check your phone number and new IMEI on their system, and update the recognized 4G IMEI to your account so that you can connect to their 4G service. After that you can move SIM to your own phone. No matter you change SIM to other 4G phones later, it still can work.
2. Some technicians might use their own method to resolve your issue. Because AT&T does not recognize your IMEM as 4G capable, so they might change your IMEI to another one which is recognized by their system, but not your current new IMEI.

After they done the setting in AT&T side, you have to use right APN, enable flight mode, and then disable flight mode to let your phone pick up mobile network coverage again. If the setting is good, your should be able to see 4G icon.

Although my phone is 4G LTE compatible, technician said there are many frequencies and bands in their LTE, so my phone might not be able to get full speed of LTE because he cannot guarantee my phone has fully support of AT&T LTE network.