I had the very same question haha, why the hell are they talking on the landline when she has a freaking mobile phone. Okay even if let's say Najma tried to call Nikhat's mobile and Nikhat didn't pick up, and then she tried her luck and called the landline, after picking up, couldn't Nikhat say alright let me call you back from your mobile?
Anyway the bigger question is, by changing her look and flirting with Farhan 24/7, what is she trying to achieve zz. I believe she wanted to make life hell for the three musketeers? I see nothing of that sort happening. Argh.