In most cases, this should fix your problem.
The default modem strings programmed inside the MD12 are set for certain countries, Canada being one of them. The default settings will not work well for many overseas countries.
The issue is that the modem is looking for a specific telecommunications line voltage when the modem is attempting to connect to a panel. Quite often the modem will have time to dial out it's number, very soon after this, the modems analysis of the line voltage comes back and says that there is no line detected, because the line voltage the modem is searching for is not present. The modem operation seizes at this point.
The DSC DLS software can override this setting and tell the modem to continue it's operation, regardless of the line voltages. This is required in New Zealand.
In the DLS software, go to the modem configuration page for the MD12 modem. There is an option which is set by default that tells the modem to monitor the line connection. Change this setting to tell it not to monitor the line connection. Apply this setting. Now you should be able to dial without any issues.
If you are not using the DLS software to dial into your panels, you will need to find out how to insert the X0 modem initialization string into the MD12 modem. The X0 string tells the modem to disregard the fact there may be no detected carrier and to continue it's operation regardless.
You may need to telnet this command to the MD12 modem.
Here is a relevant link: http://www.activexperts.com/activcomport/tutorials/modem/