The question is framed incorrectly. Islamabad, being the capital city of Pakistan has a city code. The country code of Pakistan is 92 and the city code of Islamabad is 51. To call from anywhere in the world to a person residing in Islamabad, you need to dial the country code of Pakistan first, which is +92. Next you have to dial the area code of Islamabad, which consists of 2 or 3 digits. After this you have to dial the phone number which consists of about 7 digits. In a nut-shell, the phone number of a person you are calling in Islamabad is +92(country code of Pakistan) 51(city code) XXXXXXX (phone number).