I found the solution!. I want to thank you all guys for that outpouring of support and sympathy to my post. For the record, I will like to post the solution:
Problem: on a Sierra Airbus 881 Wireless Modem, everything indicated that the SMS message was accepted for transmission, however, the message was not being sent. Solution: The Sierra Airbus 881 has 3 Com Ports!. The default com port, which I was using, just did not support SMS messaging, although it gave all the appearance of it, including interacting to AT commands. A "dmesg | grep tty" provided me with the list of USB ports that were used by the same modem. In my case, the modem was using USB0, USB1, and USB2. As soon as I changed from USB0 to USB2, it started sending the messages. Thanks CS
