There are four models of the iPhone 6 Plus phone (I've only listed the ones you will likely see). Different countries have different cellular networks. Per EveryMac:
- The Sprint/Global model supports - A1524 "CDMA EV-DO Rev. A and Rev. B (800, 1700/2100, 1900, 2100 MHz), UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz), TD-SCDMA 1900 (F), 2000 (A), GSM/EDGE (850, 900, 1800, 1900 MHz), FDD-LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29), [and] TD-LTE (Bands 38, 39, 40, 41)."
- The CDMA/Verizon model supports - A1687 "UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz), GSM/EDGE (850, 900, 1800, 1900 MHz), [and] LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)."
- The GSM/North American model supports - A1522 "UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz), GSM/EDGE (850, 900, 1800, 1900 MHz), [and] LTE (Bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29)."
As far as the chips involved:
- Qualcomm MDM9625M LTE Modem
- Skyworks 77802-23 Low Band LTE PAD
- Avago A8020 High Band PAD
- Avago A8010 Ultra High Band PA + FBARs
- TriQuint TQF6410 3G EDGE Power Amplifier Module
Depending on the model one or more of these chips are different to cover the different bands. As the IFIXIT crew flew to Australia to crack open the very first available units I suspect models they took apart was the Sprint/Global (I could be wrong).
