Iphone 4 randomly not sending messages but can receive?

Iphone 4 won't send any messages and shows red! And says not deliverd i can recive i've tryed everything i can send imessage but they say they don't get them help

Stupid question have you topped up or paid your phone bill

If you have try airplane mode in setting to reset the sim if that doesn't work turn phone off and on again

Nothing else failing ring your network provider

If you can't make phone calls sounds like either a payment issue or a network issue iPhone messages run off Wi-Fi or 3g so it doesn't actually run off the network for say

You can also download new setting or change them in the messages setting bit which may kick start it settings can be downloaded from network provider