There are fics that focus on Snape mentoring Hermonie in a more in - depth way than in the original books. This could involve him teaching her dark arts defense from a more practical and less textbook - based perspective. Another recurring theme is the idea of Hermonie being the one to truly understand Snape's pain, especially related to his unrequited love for Lily. She becomes a confidante for him, and this forms the basis of their relationship in many fanfictions.
One great place to find Hermonie and Severus Snape fanfiction is on Archive of Our Own (AO3). It has a vast collection with different takes on their relationship. Some stories might focus on a mentor - mentee relationship that could have developed further, while others explore a more romantic angle in alternate universes. Another option is Fanfiction.net, which also has a decent number of such fanfictions, though the quality can vary more widely.
Another option is a strict teacher at Hogwarts. The teacher notices that Hermione and Harry are spending a lot of time on these 'phone' conversations instead of focusing on their studies. So, the teacher tries to stop them, creating conflict in their relationship.
Well, a smut novel refers to a book where the focus is heavily on sexual themes and often has very detailed and graphic descriptions of sexual acts. It's not for everyone and is usually considered more adult-oriented.
One idea could be that they start having phone conversations secretly because they're sharing their thoughts on some difficult spells they're researching. Through these calls, they grow closer and start to realize their feelings for each other are more than just friendship.