There is nothing wrong with accessing the Internet through the numbers given on these websites and making use of their services. And there is nothing wrong with offering and accepting the rewards mentioned, and connecting them to the amount of time spent online. These rewards are regarded as a reduction in the price of accessing the Internet.
But if the rewards are greater than the price of accessing the Internet, and the aim of the one who does that is to get these rewards, that is not permissible because it is a kind of gambling. But if his aim is to access the Internet for a beneficial purpose, and the price of accessing the Internet through this company is no greater than similar services in the market, then there is nothing wrong with him accepting these rewards no matter how great they may be.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said concerning the rewards or prizes offered by some stores to those who buy a certain amount from them:
There is nothing wrong with that subject to two conditions:
(i) that the price of the product is the real price i.e., the price is not raised because of the reward or prize. If the price is raised because of the reward or prize, then this is gambling and it is not permissible
(ii) that the customer should not buy the product for the sake of acquiring the reward or prize; if he buys it for the sake of acquiring the reward only and he does not have any real need for the product, then this is a waste of money. We have heard of some people who buy a carton of milk or yoghurt when they do not want it, and they only buy it in the hope of getting the prize. So you see them buying it and pouring it away in the marketplace or at home. This is not permissible, because it is a waste of money and the Prophet (blessings and peace of Allah be upon him) forbade wasting money.
End quote from Liqa’ al-Baab al-Maftooh, 5/48; question no. 1162
Some of the scholars are of the view that these prizes or rewards are haraam, because of what they stir up of grudges and enmity and resentment they create among the store owners themselves. See the answer to question no. 22862.
But the more correct view is that this is permissible so long as the transaction is free of gambling.
And Allah knows best.