Looks like you are not a beginner in C# only, you are new to programming as well...

Here is the final code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace assignement1_nepal
{

    public class PhoneCall
    {
        public double min;
        double totalcost;
        const double unitcost = 0.10;
        
        public void NumberOfMin()
        {
            Console.WriteLine(" Enter min:");
            min = Convert.ToDouble(Console.ReadLine());
        }

        public double CalculateCost()
        {
            totalcost = min * unitcost;
            return totalcost;
        }

        public void Output()
        {
            Console.WriteLine(" total cost:{0}", totalcost);
        }

    }
    class threecalls
    {
        static void Main(String[] args)
    {
        
        PhoneCall callone = new PhoneCall();
        for(int i=0; i<3;i++)
        {
        callone.NumberOfMin();
        callone.Calculatecost();
        callone.Output();


        }
    }
    }

}

This should work. Cheers :)