Yucky Premium SMS experience

Well it would start from when i decided to switch from company X and decided to change to Vodafone. I brought a sim cad for $2, came home and activated it. I sent sms to all my friends to tell them about my new number. On the first Sunday after activating it, i got a premium sms and was angry at my account balance being robbed. It was a new number so i couldn't have given it over the internet, I didn't sign up for anything and only my friends being idiots with my number could be the only reason. I continued to get these sms every Sunday and knew it wasn't mine because the star sign it talked about wasn't even my star sign. I contacted Vodafone and they gave me contact details to premium sms company. I had to deal with them for ages and then deal back with Vodafone and threated to leave the service, return my sim card or even take it up with gov agencies. Then they gave up and contacted the sms service who elevated my case and i was contacted by them to work out a cheque which came to me a few weeks later with the wrong name! I then contacted Vodafone and then they finally gave up and credited my $12.50 to my account.
The sms stopped and a few weeks later I got an sms from Fernwood Parramatta (I am a male student) and it was then that i realised that maybe my number is owned by someone else and I am coping their subscriptions. I then started to use my phone more and regularly receive calls from friends of the ex-owner of my mobile number.

My questions were why didn't Vodafone give me my credit back in step one and take it up with the premium sms service. The other is how does the number system work and how long does a number have to be inactive before its re-sold as a new number.