Telephony - how to work with string and char

using System;
using System.Linq;

namespace Telephony
{
    public class Smartphone : ICallable, IBrowsable
    {
        public string CallOtherPhone(string Number)
        {
            if (!Number.All(ch => char.IsDigit(ch)))
            {
                throw new ArgumentException("Invalid number!");
            }

            return $"Calling... {Number}";
        }

        public string BrowseInWeb(string URL)
        {
            if (URL.Any(ch => char.IsDigit(ch)))
            {
                throw new ArgumentException("Invalid URL!");
            }

            return $"Browsing:  {URL}";
        }
    }
}