Unlikely that an attack would manifest itself in this way. I think it's an SS7 screwup since it seems that signaling is mostly what's gone pear shaped.
[edit] As others have pointed out, if b-number routing is failing, then their support sites would be flooded with people checking their account. So maybe the subscribers are in effect overloading the sites. That would make sense.
A website designed for most users to visit once per year when their contract expires likely won't scale to everyone visiting in an hour...