SMS's disappearing

We bought a Vodacom SIM card to put in our alarm system to send us SMS's when we arm, disarm or have an alarm activation. On the 17th of October I loaded a 500 SMS bundle to the SIM card. On Friday the 1st of December I noticed that I stopped receiving SMS's when I activated/deactivated the alarm, so I loaded another 500 SMS bundle which got the SMS's going again. I found this a but pecu**** that it ran out so fast so I went to count the SMS's sent to my phone number every time I do something with the alarm and found that I only received about 220 SMS's. I keep the messages send to me for insurance purposes in case there is a query to my alarms integrity. I cant disclose my phone number in question on a public forum as this links to my alarm which is a security risk.

Can you please explain to my where the remaining SMS's have gone to? I have records of all the SMS's received and they dont tally up to 500. The characters of the SMS's are very short so each SMS does not count as 2 messages