Does the time difference between Cairo and Istanbul change during the year?

Yes. Because of daylight saving time the gap moves between −1h and 0.

When is the best time to call Istanbul from Cairo?

Between 09:00 and 17:00 in Cairo, which is 09:00 to 17:00 in Istanbul.

Cairo and Istanbul are on the same local time. This gap is not fixed: daylight saving time moves it between −1h and 0 during the year.

Cairo

Egypt

--:--:--

Istanbul

Türkiye

--:--:--

A 9-to-5 day in each city overlaps for 8h. That is 09:00–17:00 in Cairo, which is 09:00–17:00 in Istanbul.

City	Time zone	UTC offset
Cairo · Egypt	Africa/Cairo	UTC+3
Istanbul · Türkiye	Europe/Istanbul	UTC+3

What is the time difference between Cairo and Istanbul?: Cairo and Istanbul are on the same local time. This gap is not fixed: daylight saving time moves it between −1h and 0 during the year.
Does the time difference between Cairo and Istanbul change during the year?: Yes. Because of daylight saving time the gap moves between −1h and 0.
When is the best time to call Istanbul from Cairo?: Between 09:00 and 17:00 in Cairo, which is 09:00 to 17:00 in Istanbul.