Considering the Evergreen campus' remote location and cement buildings, cellular reception is notoriously unreliable. There are many areas with little-to-no service; especially inside buildings (this applies for both upper campus and in housing).
Additionally, all carriers are not created equal. AT&T is generally considered to have the best reception, with Sprint in a close second. T-Mobile's reception is relatively poor and is the least reliable of the big four cellular carriers. Verizon is somewhere in between Sprint and T-Mobile. Most small and pre-paid carriers (such as Virgin Mobile and TracFone) use Sprint or AT&T's towers and as such have similar reception. That said, differences between carriers are not as drastic as this comparison implies; no carrier is superior or inferior enough to recommend switching when attending Evergreen. Reception for all carriers has been steadily improving over the last few years and should continue to do so.
Finally, there is one more issue for users of CDMA carriers (Sprint and Verizon, or anything without a SIM card). Because of the way that CDMA operates, going in and out of areas with reception will drain a phone's battery faster than staying in an area with continuous or no reception. The difference isn't massive but is very noticeable.