using Newtonsoft.Json.Linq;

{
    public class TPerson
    {
        public string FullName { get; set; }
        public string PhoneNumber { get; set; }

        public string ToJsonString()
        {
            var Obj = new JObject();
            Obj.Add("ПолноеИмя", FullName);
            Obj.Add("Телефон", PhoneNumber);
            return Obj.ToString();
          
        }
    }


    [Route("api/[controller]")]
    public class ValuesController : Controller
    {
        [HttpGet]
        public TPerson Get()
        {
            var P = new TPerson();
            P.FullName = "Vasya";
            P.PhoneNumber = "112";

            return P;
//          return P.ToJsonString(); //Вот так
        }

      
    }
}

    class TPersonConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return true;
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            if (value is TPerson)
            {
                var p = value as TPerson;
                writer.WriteStartObject();
                writer.WritePropertyName("ПолноеИмя");
                serializer.Serialize(writer, p.FullName);
                writer.WritePropertyName("Телефон");
                serializer.Serialize(writer, p.PhoneNumber);
                writer.WriteEndObject();
            }

        }
    }

    [JsonConverter(typeof(TPersonConverter))]
    public class TPerson
    {
        public string FullName { get; set; }
        public string PhoneNumber { get; set; }
    }