Need help :S

Currently my mobile is on Boost Monster TXT, the monthly $10 plan. As every other start of the month, I top-up and begin to ruthlessly get rid of the first paid 50 txt. All hell breaks loose whilst trying to send my 26th txt msg, the text fails to send and i end up with a "Message not sent, please try again alter" Few seconds alter a 113 msg is sent with the following

" Sorry, you have insufficient fnds to send this message. To top up, call 0800323232 from your mobile phone. " yadayada.

Thinking I had accidentally sent a txt msg to 021, I called *333 to make sure, only to find that I still have 1975 text message left on boost text, with $5.00 credit!

What is going on? Has anyone else experienced this problem?

My mobile is an old Nokia 2118, in good condition. Nice handy phone, never had problems with it, never dropped it into water or anything. I have called the telecom call center many times. Always end up talking to someone overseas! Not liking it one bit. :(

Could something like being blocked from recieving chargeable sms txt cause this? Just a thought.

Any Advice, Information or whatever would be helpful :)

Thanks!!!