You can override the default Voice Codecs in the Outgoing VOIP Call Routing rule. To do it follow these steps:
1. Open Fax Voip T.38 Modem Control Panel.
2. Click VOIP in TreeView, then click Outgoing Plan.
3. To create new rule click New. To edit an existing rule select it and click Edit (or double-click the selected rule).
4. In the VOIP Outgoing Call Routing Rule dialog switch to the Codecs panel.
5. To be able to change the default Voice Codecs configuration, uncheck Use Default option.
6. To add a codec, check it in the Voice Codecs list.
7. To remove a codec, uncheck it in the Voice Codecs list.
8. Use <Up> and <Down> buttons to change preference order for selected audio codecs. You should select one of the entries in the Voice Codecs list before using these buttons.
9. Click <OK> button to fix changes.
10. Click <Apply> button to save modified settings for rules. Do not forget to restart Fax Voip T.38 Modem to operate with new settings.