I had same problem and solved. Refer below codes.

PBoolean MYExternalRTPChannel::OnSendingPDU(H245_H2250LogicalChannelParameters& param) const
{
 // SONY

 if(payloadCode>=96)
 {

  param.IncludeOptionalField(H245_H2250LogicalChannelParameters::e_dynamicRTPPayloadType);
  param.m_dynamicRTPPayloadType = payloadCode;

  param.IncludeOptionalField(H245_H2250LogicalChannelParameters::e_mediaPacketization);
  param.m_mediaPacketization.SetTag(H245_H2250LogicalChannelParameters_mediaPacketization::e_rtpPayloadType);
  H245_RTPPayloadType & rtpPacketization = param.m_mediaPacketization;
  rtpPacketization.m_payloadDescriptor.SetTag(H245_RTPPayloadType_payloadDescriptor::e_oid);
  ((PASN_ObjectId &)rtpPacketization.m_payloadDescriptor) = OpalPluginCodec_Identifer_H264_Aligned;//OpalPluginCodec_Identifer_H264_Generic;

  rtpPacketization.IncludeOptionalField(H245_RTPPayloadType::e_payloadType);
  rtpPacketization.m_payloadType = payloadCode;
 }
 else
 {
 }

 return H323_ExternalRTPChannel::OnSendingPDU(param);
}