Configuring a trunk group.
Both SwyxWare and the Asterisk server providing the service showed the trunk as down. We eventually traced the problem to the fact that our remote Asterisk server was connected via a VPN which had been set up and started after the Swyx software was already installed. Because of this, Swyx was not listening on the IP address allocated to our VPN, and so couldn't communicate via the VPN.
To get round this issue, we switched to a local Asterisk server, set up with the same test account, and re-configured SwyxWare accordingly. However the trunk failed to register with the account, and was unable to place calls until the Asterisk server was configured to authenticate the incoming connection by source address instead of by SIP registration. This got the trunk working, but only if the Swyx configuration was set to register the SIP connection with the outgoing server. If that option was de-selected, as would be normal with this form of authentication, then the trunk failed again. Trusted source address authentication of this sort is secure and stable, but requires the source to have a static IP address. This won't be an issue for most small businesses, but is worth bearing in mind.
