on mobile phones if the number doesn't start with 0 it seems to be spam unless you subscribed - then the message is the same - you pay for any sort of response.
They were designed for subscriptions and competitions but I get a few spam and I am not on any directory and no-one like that has the number - they just ry all numbers and the Networks refuse to acknowledge there is anything they can do about it. I can - not use the network as much, and keep the phone switched on so there is a cost to them