When I was in process of selling my ex-cars, and had posted the ads on OLX and Pakwheels only, I used to get messages from not-so-famous websites companies that please also post ad on our website, with their marketing details, etc.
I can even name them, but would not like to get in to some controversy, so not mentioning.
I always deleted such text messages.
Getting contact nos. from OLX, Pakwheels is not a big issues. When they get contact nos. sending of messages starts.
And if there is a link in such SMS, which asks for the consent, and you click it, a similar ad with similar details will get posted on their websites (either automatically by picking details from OLX, Pakwheels, etc. or a person manually putting in the data), and it may become painful later.