What should a remorseful woman do who spoke with a strange man on the phone during her 'iddah to atone for her action?

Speaking on the phone during the iddah period is subject to the same rulings as other times: what is good is good, and what is bad is bad. If the speech is unlawful, the speaker must repent. However, if it is permissible, such as inquiring about well-being or offering condolences, then there is no blame upon her.