Some Gateways do not detect a busy tone, dial tone or hangup correctly. This is often caused because the device is pre-configured for use in the USA, and does not understand the call progress tones of your country. In this case you will need to configure the Call Progress Tones for your country.
This web site has a database of Call Progress Tones for all Countries
There is also a link to the exact string that you should enter for Linksys/Sipura devices. Please contact the VOIP Gateway manufacturer for further help on configuring your gateway with the correct Call Progress Tone settings.