What is the ruling on speaking with one's brother's wife over the phone regarding some life matters, within the bounds of decorum, after repenting from an illicit relationship with her?

This forbidden relationship with your brother's wife is evil and reprehensible, involving sin and the severing of kinship ties. Its cause is often laxity in the brother entering upon his brother's wife. You have done well by feeling remorse and returning to Allah, so make it a sincere repentance, fulfilling all its conditions. There is no harm in speaking with her out of necessity, within the bounds of Islamic law, as long as you do not feel any temptation towards her. Otherwise, avoid her completely to block the means (to sin).