I find it is possible to call some numbers on my phone without being billed. Can I call these numbers?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We advise you to avoid the practice mentioned in your question as those bills might be registered on the account of another subscriber or that the authorities might discover this practice and then they might create problems for you, or for some other people, or for Muslims in general. Indeed, a Muslim is always asked to abide by faithfulness, fulfilling the promise and avoiding unlawful money.

Allah knows best.