My samsung galaxy 5s started sending me the same message yesterday, saying it came from myself! At first i thought i was maxed out on text so i deleted quite a few but nothing new came through. I sent a message to myself from my other phone number and it came through fine so i deleted the "insufficient..." message. Until it came across again today (twice 2 mins apart) so i stopped at my local verizon store they had no idea so no issues with my account gave me the 800 number to call, but they have no idea either. They fear it may be a phishing scam so advised me just to delete if anymore come across while they investigate! You mentione you recently "blocked" a number, so did i! Maybe there is some connection?