There are slightly varying opinions regarding this, some more strict, and some perhaps a bit more lenient, so I cannot say that this is the only way and you must stick to it.  InshaAllah more resources regarding the matter will be posted soon.
Is it Permissible to Talk to ones Fiancée over the
Answered by
the Muhadith, the ‘Allaama, Shaykh of Hadeeth
Muhammad Nasr ud-Deen al-Albaani
Translated by
Abbas Abu Yahya
This is a translation of the transcript of a question that was asked to Shaykh
al-Albaani – may Allaah have mercy upon him.
Shaykh: Yes
Questioner: Assalamu alaykum
Shaykh: wa Alaykum Assalamu wa Rahmatullaahi wa Baraktuhu
Questioner: If you don’t mind is the noble Shaykh al-Albaani there?
Shaykh: He’s with you.
Questioner: Good, if you would allow me O Shaykh I have some questions
to ask.
Shaykh: Go ahead.
Questioner: Is it permissible to talk to my fiancée over the phone?
Shaykh: have you contracted the marriage yet or not?
Questioner: not yet.
Shaykh: It’s not permissible.
Questioner: Not permissible??
Shaykh: Not allowed.
Questioner: Even if it’s for advice?
Shaykh: it’s not permissible.
Questioner: ok is it permissible for me to visit her and sit with her if the
Mahram is present?
Shaykh: with a Mahram being present and she comes in front of you wearing
a Jilbaab in Hijab, like when she goes out, then it’s allowed, otherwise no.
Questioner: is it possible for her to uncover her face?
Shaykh: it’s possible, if it’s only the face.
Questioner: only the face?
Shaykh: She shouldn’t wear a beautified dress and a short dress etc.
Questioner: ok regarding sitting with her, what is permissible for me to talk
to her about?
Shaykh: Do not talk to her except with what you would talk to with other
than her.
Questioner: Ok if she asks me for a picture of me, is it ok to give it to her or
Shaykh: just like if you asked her for her picture.
Questioner: yes??
Shaykh: I said just like if you asked her for her picture.
Questioner: yeah.
Shaykh: Is it permissible?
Questioner: No.
Shaykh: and my answer is also no.
Questioner: your answer is no??
Shaykh: no, definitely no.
Questioner: about what??
Shaykh: About what! For the same thing what you said, that you cannot ask
her for her picture.
Questioner: yeah.
Shaykh: understand?
Questioner: yeah, yes.
Shaykh: If you understand then stick to it.
Questioner: But O Shaykh sometimes a person is forced to phone her, is
this permissible?
Shaykh: I don’t think there is a need, you want to marry her don’t you?
Questioner: For example, is it permissible to phone her for the possibility
that I can visit her, at such and such time?
Shaykh: why do want to visit her?! What’s the difference between her and
any other woman?
Questioner: Do you mean it’s not allowed to visit her?
Shaykh: O my brother, I say to you what’s the difference between her and
any other woman? Why do you want to visit her? You want to marry her;
you marry her by requesting it from her guardian.
Questioner: If her guardian is present?
Shaykh: You want to marry her, you marry her by requesting her guardian, if
there is an original agreement then you can visit her if the guardian is
present, to see her and she sees you, as for visiting her then no!
Questioner: It’s still not allowed to visit even after the engagement?
Shaykh: After the engagement?
Questioner: Yeah.
Shaykh: She remains to be a stranger to you O brother until you perform
the marriage contract.
Questioner: Thank you, may Allaah reward you O Shaykh.
Shaykh: And you.
Questioner: May Allaah be generous to you.
Shaykh: May Allaah protect you… Sallamu alayk.
Questioner: Assalamu alaykum
Shaykh: Wa Alaykum Assalamu wa Rahmatullaahi wa Baraktuhu
Taken from: Silsilah Huda wa Noor, tape no. 269 at 10mins.
All Praise belongs to Allaah, may His peace and blessings be upon our final Prophet Muhammad,
his family, his Companions and all those who follow his guidance.
Shaykh Saalih al-Fawzaan (may Allaah preserve him) was asked about a man speaking to his fiancée over the phone – is that permissible according to sharee’ah or not?

He replied:

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.