I am an engaged girl, which means that i`m not officially married yet (through signing a marriage contract), but we made the initial agreement (Fatihah) with the consent of all parties. My question is: Is it permissible for me to sit with my fiancé wearing my Hijab, and to talk with him through the phone before getting married?

Jazakum Allahu Khairan.

Praise be to Allah, prayers and peace be upon His kind Messenger,

Yes, you can sit with your fiancé with your Hijab on, but one of your Maharam males (like your father, brother, uncle, etc) should be preazasent with you. Also, you can talk with him through the phone, but briefly with the purpose of getting more acquainted, and not just for chatting. This, howver, should be for a short and a limited time, because marriage might not happen in the end, then harm might be induced on you, in particular, since too much talking may reveal something about your private life which no one has the right to know except you. Therefore, I advise you not to talk too much to keep yourself safe from harm. I pray to Allah to make success and prosperity your permanent companion in this worldly life and in the Hereafter, and praise be to Allah, Lord of the worlds.