Actually, that's not that surprising. The carriers put QoS (Quality of Service) on the traffic on their networks. Voice gets the highest priority, then they'll have various levels of other traffic (http, encrypted/SSL, video (H323, Skype, FaceTime). But, here's the funny thing - SMS / text has always had a "fire and forget it" setup. Messages historically were never guaranteed by any service contracts to be delivered and the servers would often purge messages without getting acknowledgements from their clients. SMS was always the least important type of traffic. So, it stands to reason in areas of high congestion that it'll be the least reliable means of communicating (which is also why BBM - BlackBerry Messenger took off in its day - delivery was guaranteed).