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.
It is not permissible for you to talk with this girl over the phone especially that you mentioned that this leads you to silly conversations.
Nonetheless, if you love this girl and think that she could be a pious wife if you marry her, then you should propose to her in a legal way and then stop talking to her as she is still a non-Mahram woman to you until you marry her. Afterwards, you should seek the Help of Allah and endeavor to acquire the wedding requirements and marry her. Then, you should both endeavor to become pious and abide by the Sharee’ah of Allah till you build together a home of ‘Eemaan (faith) in which you nurture a pious generation of believers.
Allah knows best.