‘this’ argument has type const but function is not marked const

If you want to set a value, use the set method. get Methods are only to obtain variables, not to set the inner variables of a class (If they are defined the way you did).

The correct usage is:

Customer* CreateCustomer(const string& id, const string& name, const string& address) {
    Customer* temp = new Customer();

    temp->set_PhoneNumber( id );
    temp->set_Name( name );
    temp->set_Address( address );

    return temp;
}

Also, you have to alter the interface of your methods:

class Customer {

private:
    string PhoneNumber_;
    string Name_;
    string Address_;

public:
    string get_PhoneNumber() const {return PhoneNumber_;} // Accessor
    void set_PhoneNumber(const string& x) {PhoneNumber_ = x;} // Mutator

    string get_Name() const {return Name_;}
    void set_Name(const string& x) {Name_ = x;}

    string get_Address() const {return Address_;}
    void set_Address(const string& x)  {Address_ = x;}
};

Since you want to set strings and not numbers.

Using const string& as function arguments is better than string to not copy the string when passing it as an argument. Since it is a const reference, you don’t have to fear the function could manipulate the input.