There are guidelines that have to do with speaking to one’s fiancée, details of which may be found in Question no. 13791.
If these guidelines are met, especially the stipulation that there be no risk of provoking desire and if there is a need to speak to her, then it is permissible. But if any of these guidelines is not met then talking to her becomes something that is not allowed according to sharee’ah. If you talk to her and one of these guidelines is not met whilst you are fasting, this is going to affect your fast. In this case it is worse and a more serious sin, because it is happening at a time of virtue, and because the one who is fasting has to protect his fast from anything that may spoil it or affect it.