You can use this (GSMCOMM) library, please consider for sending long messages you have to create multiple pdu mesage this link, please download it, the packages contains sample and information enough for your problem
it support also GSM modems.
SMS libraries (for GSM phones)[
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Sample
#region MultipleMessaging
private SmsSubmitPdu[] CreateConcatMessage(string message, string number, bool unicode, bool showParts)
{
SmsSubmitPdu[] pdus = null;
try
{
if (!unicode)
{
pdus = GsmComm.PduConverter.SmartMessaging.SmartMessageFactory.CreateConcatTextMessage(message, number);
}
else
{
pdus = GsmComm.PduConverter.SmartMessaging.SmartMessageFactory.CreateConcatTextMessage(message, true, number);
}
}
catch (Exception ex)
{
return null;
}
return pdus;
}
private void SendMultiple(SmsSubmitPdu[] pdus)
{
try
{
foreach (SmsSubmitPdu pdu in pdus)
{
mainComminucator.SendMessage(pdu);
}
}
catch (Exception ex)
{
}
}
#endregion