The SMS callback module provides an interface based on http (protocol used in web) for receiving parameters containing body of SMS and the phone number from which the SMS has been sent. The names of the parameters can be freely modified by the administrator. This method allows to connect any equipment which is capable of forwarding SMS messages over Internet via http, such as GSM gateways. Instead of purchasing a GSM gateway you can also rent an SMS number from a provider and have the incoming messages delivered via Internet to specified URL. Such service is often called SMS on demand.
Other method is to use a mobile phone (like for example Samsung, nokia ) connected to a PC through USB cable. Our company provides special application called SMSreader which reads incoming messages from the phone and then forwards them to defined URL of voipswitch's server. There is no special hardware requirements for this application, it can be a mere PC with any version of Windows OS. Also the internet link does not have to be high speed as the SMS messages need very little bandwidth. Such setup can be located anywhere in the world what is very useful if you want to provide local GSM numbers for this service in various countries. Many access numbers can be connected to one VoipSwitch server.