I have recently started receiving various spam text messages. These are coming from various sources. Some appear to come from mobile numbers, some from short sms codes and some from the internet. Whilst I have been replying to the sms messages with stop. The ones from the internet I am unable to reply to. They are usually just web links that appear in these messages. When I receive one of these messages (which I believe are from the internet) it has a picture of a globe rather than a picture of an envelope. I have also realised I am being 10p per message being received, whilst this does not break the bank, don't see why I should have to pay. As far as I'm aware I have not signed up to any service. Maybe this is a coincidence, but I have had the same phone number for 10 years and been on T-Mobile, Orange, O2 and recently moved to Vodaphone. The problems have only happened since I moved to Vodaphone.
This is just like Spam e-mail. By returning the message you are validating your phone number and inviting more messages. I think you should talk to your carrier about blocking those inbound numbers. Length of time at your Number makes no difference if you are the victim to a "demon dialer".
Hear what you are saying about returning the message. I had already replied STOP ALL to the senders before reading you post. Since then (touch wood) I have not recieved any more. Any more then think I will be contacting Vodafone to but a block on it. Interestingly, on their own website people complain about having problems with vodafone which they have not had with other networks.