Praise be to Allah.
It is not permissible except in a case where both parties trust one another and where the parents agree to the marriage and have no objections. In this case it is acceptable for them to speak to one another in a normal manner, discussing matters of life that concern them. But if they know that their parents do not agree to that, then it is not permissible for them to speak to one another in this case.
Shaykh ‘Abd-Allaah ibn Jibreen.
The fiancée is still a non-mahram woman and speaking to her is still speaking to a non-mahram woman, so it must be done properly and only as much as is necessary, such as agreeing on specific matters to do with after they get married. Attention must also be paid to the following:
1 – It must be done with the agreement of the woman’s guardian and with no objection on his part to the marriage.
2 – The talk should not involve anything that may provoke desire or cause temptation.
3 – There should be no other way of telling her what he wants to say, such as through his sister or her brother, or by letter.
4 – It should not go beyond what is essential.
And Allaah is the Source of strength.