said by NOCMan:Text messaging uses the SS7 network plus, the page and sync channels of cellular towers. Page and Sync channels are what keeps your phone on the network when you're not on a call. When someone calls you the P&S channel pages your phone and sets the call up on a channel. There are only so many P&S channels available per tower and as text messages increase you begin to run into hard limits.
There used to be a method by which one could use the GPRS data channel to send text messages on the ATT (and other GSM providers) network and avoid that problem... Problem is that, last I checked, most of the providers had blocked that ability for whatever reason.
