Actually, there is no truly global full band phone that covers every 2G / 3G
/ 4G cellular frequency / band in the world, although the modems inside
many top tier cellular phones are capable of supporting all.
The phone supporting the most 2G / 3G / 4G frequencies / bands is the Sprint
version of iPhone 6 / iPhone 6 Plus (A1586 / A1524). It supports all 2G /
3G / 4G frequencies / bands used by all four major carriers in the US and
all three major carriers in China, except T-Mobile 4G LTE 700 MHz / band 12
(actually, even the T-Mobile version doesn’t support that). And it also
should support all 2G GSM frequencies in Europe, but not sure about 3G / 4G.
The next best thing, in terms of frequency / band support, is the Motorola
Google Nexus 6 North America version (XT1103). It supports all 2G / 3G / 4G
frequencies / bands for all four major carriers in the US; but it only
supports one out of four 4G LTE frequencies / bands used by three major
carriers in China (that is, 2500 MHz / Band 41, although it is used by all
tree Chinese carriers), and two of three 2G / 3G CDMA frequencies used by
China Telecomm (中国电信), besides the support of all 2G / 3G GSM / WCDMA
frequencies used by the other two Chinese carriers, China Mobil (中国移动)
and China Unicom (中国联通).
Then, there is this soon to be released Asus ZenFone 2, although it is a GSM
phone, meaning it does not support any 2G / 3G CDMA frequencies, so it
cannot be used for China Telecomm (nor Sprint and Verizon), at least 2G / 3G
wise, since China Telecomm is a CDMA carrier (so do Sprint and Verizon). It
supports all or most of 2G / 3G GSM / WCDMA frequencies and some of 4G LTE
frequencies / bands used by two major US GSM carriers (ATT and T-Mobile);
and all 2G / 3G GSM / WCDMA frequencies used by China Mobile and China
Unicom, as well as all 4G LTE frequencies / bands used by all three Chinese
carriers.
Every other phones, even the latest and top tier ones, so far, AFAIK, only
support / cover a small sub set of 2G / 3G / 4G frequencies / bands, and are
carrier or region dependent.