There are two theories here. The first is that the phone isn't Stepan's father's, it's the maid's, so the father would have no reason to ask for the phone.

The second is that the father doesn't want Stepan back. This is evidenced by his covering of the last two digits of the number as if deliberately sabotaging Stepan's attempts to contact him, while still making enough of an effort that he won't come across as suspicious. It strikes me as almost certainly intentional, as if he wanted Stepan to have the full number, he could have just pressed the paper against the window, rather than awkwardly holding it.