Serializing and Deserializing object in C# without using file system

Disclaimer: I don't have anything against the use of Newtonsoft Json in general

The problem is that you are using StreamWriter to write the re-constructed bytes from Convert.FromBase64String to a MemoryStream.

MSDN: (my empasis)

Implements a TextWriter for writing characters to a stream in a particular encoding.

In your case this results in a much smaller buffer in MemoryStream thus leading to an exception later in Deserialize.

We can see the differences in sizes below:

MemStreamTest.exe Information: 0 : Position: 206 after save
MemStreamTest.exe Information: 0 : ToArray returned: 206 bytes
MemStreamTest.exe Information: 0 : Position: 13 after reconstruction

Change this:

public Stream GenerateStreamFromString(string s)
  {
    MemoryStream stream = new MemoryStream();
    StreamWriter writer = new StreamWriter(stream);
    writer.Write(Convert.FromBase64String(s));
    writer.Flush();
    stream.Position = 0;
    return stream;
  }

...to:

public Stream GenerateStreamFromString(string s)
{
    MemoryStream stream = new MemoryStream();
    var bytes = Convert.FromBase64String(s);
    stream.Write(bytes, 0, bytes.Length);
    Trace.TraceInformation($"Position: {stream.Position} after reconstruction");
    stream.Position = 0;
    return stream;
}

The following example of Call results in 206 bytes in each MemoryStream.

var call = new Call {PhoneNumber = "0812345678", Duration = TimeSpan.FromMinutes(5)};

[Serializable]
public class Call
{
    public TimeSpan Duration { get; set; }
    public string PhoneNumber { get; set; }
}

Results:

MemStreamTest.exe Information: 0 : Position: 206 after save
MemStreamTest.exe Information: 0 : ToArray returned: 206 bytes
MemStreamTest.exe Information: 0 : Position: 206 after reconstruction

Complete listing

using System;
using System.Diagnostics;
using System.IO;
using System.Runtime.Serialization.Formatters.Binary;

namespace MemStreamTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var program = new Program();
            program.Run();
        }

        private void Run()
        {
            var call = new Call {PhoneNumber = "0812345678", Duration = TimeSpan.FromMinutes(5)};
            var contents = SerializeObject(call);

            var other = deSerialize(contents);
        }

        public string SerializeObject<T>(T objectToSerialize)
        {
            BinaryFormatter bf = new BinaryFormatter();
            MemoryStream memStr = new MemoryStream();
            try
            {
                bf.Serialize(memStr, objectToSerialize); // serialise as binary
                Trace.TraceInformation($"Position: {memStr.Position} after save");
                memStr.Position = 0;
                var bytes = memStr.ToArray();
                Trace.TraceInformation($"ToArray returned: {bytes.Length} bytes");
                
                return Convert.ToBase64String(bytes); // binary to Base64
            }
            finally
            {
                memStr.Close();
            }
        }

        public Call deSerialize(string Serialized)
        {
            Stream Fs = GenerateStreamFromString(Serialized);
            BinaryFormatter F = new BinaryFormatter();
            Call s1 = (Call)F.Deserialize(Fs);
            Fs.Close();
            return s1;
        }

        public Stream GenerateStreamFromString(string s)
        {
            MemoryStream stream = new MemoryStream();
            var bytes = Convert.FromBase64String(s);
            stream.Write(bytes, 0, bytes.Length);
            Trace.TraceInformation($"Position: {stream.Position} after reconstruction");
            stream.Position = 0;
            return stream;
        }
    }

    [Serializable]
    public class Call
    {
        public TimeSpan Duration { get; set; }
        public string PhoneNumber { get; set; }
    }
}

Final thoughts

Unless you really want a Base64 payload you would be advised to use Newtonsoft Json.