With ref to https://danielwertheim.se/csharp-always-valid-value-objects/

VO.cs

namespace MyApp.Domain
{
    using Ardalis.GuardClauses;

    public class CountryCode
    {
        public CountryCode(string value) => Value = Guard.Against.UnknownCountryCode(value, nameof(value));

        public string Value { get; }
    }

    public class Extension
    {
        public Extension(string value) => Value = Guard.Against.InvalidExtension(value, nameof(value));

        public string Value { get; }
    }

    public class AreaCode
    {
        public AreaCode(string value) => Value = Guard.Against.UnknownAreaCode(value, nameof(value));

        public string Value { get; }
    }

    public class PhoneNumber
    {
        public PhoneNumber(CountryCode countryCode, AreaCode areaCode, Extension extension)
        {
            CountryCode = countryCode;
            AreaCode = areaCode;
            Extension = extension;
        }

        public CountryCode CountryCode { get; }
        public AreaCode AreaCode { get; }
        public Extension Extension { get; }
    }
}

namespace TestProject2
{
    using System;
    using FluentAssertions;
    using MyApp.Domain;
    using Xunit;

    public class UnitTest1
    {
        [Theory]
        [InlineData("A", "1", "2")]
        [InlineData("1", "A", "2")]
        [InlineData("1", "2", "A")]
        public void All_parts_exceptions_on_invalid(string countryCodeValue, string areaCodeValue, string extensionValue) =>
            FluentActions
                .Invoking(() =>
                {
                    var countryCode = new CountryCode(countryCodeValue);
                    var areaCode = new AreaCode(areaCodeValue);
                    var extension = new Extension(extensionValue);
                    return new PhoneNumber(countryCode, areaCode, extension);
                })
                .Should().Throw<Exception>();
    }
}

namespace Ardalis.GuardClauses
{
    using System;
    using System.Linq;

    public static class CountryCodeGuard
    {
        private static readonly string[] KnownCountryCodeValues = { "44", "01" };
        private static readonly string Message = $"Should be one of {string.Join(", ", KnownCountryCodeValues)}";

        public static string UnknownCountryCode(this IGuardClause guardClause, string input, string parameterName)
        {
            if (!KnownCountryCodeValues.Contains(input))
            {
                throw new ArgumentException(Message, parameterName);
            }

            return input;
        }
    }
}

namespace Ardalis.GuardClauses
{
    using System;
    using System.Linq;

    public static class AreaCodeGuard
    {
        private static readonly string[] KnownAreaCodeValues = { "0131", "0141" };
        private static readonly string Message = $"Should be one of {string.Join(", ", KnownAreaCodeValues)}";

        public static string UnknownAreaCode(this IGuardClause guardClause, string input, string parameterName)
        {
            if (!KnownAreaCodeValues.Contains(input))
            {
                throw new ArgumentException(Message, parameterName);
            }

            return input;
        }
    }
}

namespace Ardalis.GuardClauses
{
    using System;
    using System.Linq;

    public static class ExtensionGuard
    {
        public static string InvalidExtension(this IGuardClause guardClause, string input, string parameterName)
        {
            if (input.Length != 4)
            {
                throw new ArgumentException("Should be length of exactly 4", parameterName);
            }

            if (!input.All(char.IsDigit))
            {
                throw new ArgumentException("Should contain only digits", parameterName);
            }

            return input;
        }
    }
}