I am glad that you decided to talk to your partner about what was bothering you. It is a good thing that you have taken initiative and started working towards improving your communication. I understand that you want to have good talks and understand each other well before you get married.
As you have mentioned earlier that this is something new, and that your partner has a shy nature, it will take time for her to get comfortable with everything. If you feel that she is not comfortable with calls, try sending her messages and ask her before you call. Leave her messages saying that you hope her day goes well, or something similar.
As I have mentioned earlier, it takes time for a person of shy nature to come out and talk or interact openly. So expecting your partner to immediately change their nature would not be right. You can keep reminding your partner that she can share anything she wants to with you and you will be there for her. It can also be that she may want to talk with you, but her shy nature may not be allowing her to.
Again, as mentioned earlier, being with her and showing that you care will help.