the object has type qualifiers that are not compatible with the member function

Question

#include<iostream>

using namespace std;

class PhoneNumber

{

    int areacode;
    int localnum;
public:

    PhoneNumber();
    PhoneNumber(const int, const int);
    void display() const;
    bool valid() const;
    void set(int, int);
    PhoneNumber& operator=(const PhoneNumber& no);
    PhoneNumber(const PhoneNumber&);
};

istream& operator>>(istream& is, const PhoneNumber& no);


istream& operator>>(istream& is, const PhoneNumber& no)
{

    int area, local;
    cout << "Area Code     : ";
    is >> area;
    cout << "Local number  : ";
    is >> local;
    no.set(area, local);
    return is;
}

at no.set(area, local); it says that "the object has type qualifiers that are not compatible with the member function"

what should i do...?