I wanted to know to know how the wives of Nabi SAW used to call upon him, I mean the title they were using, like we they calling using his name, "Muhammad" ?
In a hadith where the Prophet () discribes how he know that his wife 'Aisha () was angry with him (for what ever reasons) he says:
... "When you are pleased with me, you say, 'No, by the Lord of Muhammad,' but when you are angry with me, then you say, 'No, by the Lord of Abraham.' " I (refering to 'Aisha) said: Allah's Messenger, by Allah, I in fact leave your name (when I am annoyed with you). ... (sahih al-Bukhari and sahih Muslim)
From this we can conclude that his wives have been calling him either by his title (Allah's Messenger/Rassul Allah رسول الله) or by his name Muhammad محمد.
And here are some other ahadith showing that his title was the most used in conversations with his wives:
From sunan abi Dawod (Again a conversation with 'Aishah):
We entered upon A’ishah, wife of the Prophet (ﷺ). She said The Apostle of Allaah (ﷺ) entered upon me and saw two silver rings in my hand. He asked What is this, Aishah? I said I have made two ornaments myself for you, Messenger of Allah (ﷺ). He asked Do you pay zakat on them? I said No or I said Whatever Allah willed. He said this is sufficient for you (to take you) to the Hell fire.
And a hadith statment which has been quoted by a couple of his wives for example in al-Muwatta' (~on the authority of Umm Salamah), in jami' at-Tirmidhi (on the authority of 'Aishah), in sahih al-Bukhari, sunan ibn Majah and sahih Muslim (on the authority of Zaynab bint Jahsh) all of them asking the Prophet () the same question:
...'O Messenger of Allah! Will they be destroyed while they are righteous among them?...