Error: "undefined external error" (after Windows 10 update 1803).

Fix microphone permission in the Windows settings (Windows Settings => Privacy => Microphone).

Error: "Unable to find default audio device".

Speakers and microphones are required. To make calls you must have input and output sound device in your system. Same for RDP connections.

Error: "An invalid parameter was passed to a system function".

Allow access to the microphone in Kaspersky Anti-virus settings.

Error: "End of file".

Change Transport from Auto to UDP.

Can't make or end a call

• Make sure your SIP account configuration is correct. Check fields: domain, username, password, proxy, server, login, transport.
• Make sure you dial the correct number and in the correct format, with the correct prefix, etc (often not found error).
• Disable SIP ALG in the router settings
• Test with a clean installation of Vantact Softphone, where all additional features are disabled by default
• Try with/without "Allow IP rewrite". Try with/without STUN server.
• Try to set the source port in settings to 5060
• Try calling from another computer, using a different router or other internet connection.
• Check your PBX configuration, NAT support.

No sound or one-direction sound

This may happen if you use one or more routers (with NAT) on the way to the PBX. Search the Knowledgebase for your specific router, and turn off SIP ALGs.