Your mobile service provider will be able to tell you the origins of the short code used (87070). Legally sending STOP and nothing else to the number should remove you from their mailing list. Doesn't explain how they got your number in the first place though. I read recently that mobile phone companies recycle numbers particularly when someone gets a new phone and doesn't keep their old number. Someone could then get a phone with the old number. If the previous owner had signed up to a service like this it could possibly continue. I think the service provider is required to do something to remove the number from these sort of lists but some fall through the net.