Call Progress tones are audible tones used  by the public switched telephone network (PSTN) central office or a private branch exchange (PBX) to signal calling parties the status of phone calls. VoIP Gateways must understand these Call Progress Tones in order to detect a busy tone, dial tone or hangup correctly.

Unfortunately these Call Progress Tones can vary per country, and in some cases you need to configure your VoIP gateway accordingly. Many analog devices are pre-configured for use in the USA, and do 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

http://www.3amsystems.com/wireline/tone-search.htm

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 for your country