PhoneAuthentication is received as null.

Code used to add Authentication is below: 

orignalSigners.IdCheckConfigurationName = "Phone Auth $";
                                    RecipientPhoneAuthentication recipientPhoneAuthentication = new RecipientPhoneAuthentication();
                                    List<string> senderProvidedNumbers = new List<string>();
                                    recipientPhoneAuthentication.SenderProvidedNumbers = senderProvidedNumbers;
                                    recipientPhoneAuthentication.SenderProvidedNumbers.Add(t.PhoneNumber);
                                    orignalSigners.PhoneAuthentication = recipientPhoneAuthentication;

Trying to get Signer with Authentication using following code but the signer.PhoneAuthentication is received as null:

System.Net.ServicePointManager.SecurityProtocol = (System.Net.SecurityProtocolType)3072;
                accountID = this.GetAccountID();
                EnvelopesApi envelopesApi = new EnvelopesApi(apiClient);
                envelopesApi.ApiClient = apiClient;
                Recipients results = envelopesApi.ListRecipients(accountID, envelopID);
                Envelope envelop = envelopesApi.GetEnvelope(accountID, envelopID);

                signerList = results.Signers;
                dict = new Dictionary<Envelope, List<Signer>>();
                dict.Add(envelop, signerList);

Please help me figure out what I am doing wrong while reading the recipients