I am using ActiveSMS with a GSM device (GSM Modem / GSM Handset).

However I am getting a failure when I try to send SMS messages.
SMS message send can fail for a number of reasons, here is the list of the most common issues:

1) The phone number is in the correct format. You will need to specify the phone number in international format, starting with your country code (e.g. for the UK 447xxxxxxxxx). You can setup ActiveSMS to automatically add the international dialling code. On the Transports tab, click the 'International Dialling Rules...' button, enable dialling rule and enter 'XX' as the code.

2) The GSM Device (GSM Modem/Mobile Handset) has insufficient mobile network signal strength. The red LED on a GSM modem will often flash when GSM Modem has successfully registered on to the network.
Resolution: Reposition Antenna to obtain adequate signal strength.

3) SIM Card is not valid. For example, pay-as-you go SIM has insufficient credit.
Resolution: Please test SIM card is valid by inserting into a mobile phone and send a message manually. Contact your operator if this test does not work.

4) The default SMSC Number is incorrectly configured.
Resolution: Find out the correct SMSC number for your network. Enter the SMSC Number in the transport setting in the ActiveSMS Console