Praise be to Allah.
There is nothing wrong with corresponding with your fiancée in order to agree on matters to do with the wedding, if that is done with the knowledge and supervision of her parents, and it is free of any expressions of love that are not permissible between a man and a non-mahram woman. It is well known that the man is a “stranger” (non-mahram) to his fiancée until the marriage contract is done.
It makes no difference whether this correspondence is done via e-mail or regular mail, or over the telephone, but it is better if the correspondence and conversation is with her guardian only.
Shaykh Saalih al-Fawzaan (may Allah preserve him) was asked about a man speaking to his fiancée over the phone – is that permissible according to sharee’ah or not?
There is nothing wrong with a man speaking to his fiancée over the phone, if that is after they have agreed to marry, and the conversation is in order to discuss different issues and is done only as much as is needed, and there is no fitnah involved. If that is done through her wali (guardian) that is preferable and less dubious.
End quote from al-Muntaqa, 3/163
And Allah knows best.