The reason they've spent less on every quarter since the iPhone's launch is that since the end of 2007 credit's been constricting and businesses have been uncertain about their future economic outlooks. Companies like AT&T utilize short term credit and long term debt to expand.
Cash and Cash equivalents may increase, but thats because companies have to protect their liquidity in recessions like this. Companies that don't have idle liquidity can get crushed in a matter of days. If AT&T ended up locked out of commercial paper markets, they could burn through that 4.375 billion in a matter of days.
Also, increasing cash and cash equivalents could easily be a sign that AT&T is either preparing to: buy another provider, or build more capacity into the network. They may just be waiting for more clear signs that consumer spending is recovering.
2008 and 2009 were not good years to be shoveling cash out the door for long term investments.