The VTA1000 is a SIP/H323/Skype VoIP Gateway for Dialup/WiFi/Broadband sold by PCPhoneline.com.
Their website explains how to setup the driver on the PC end, but doesn't explain what to do on the Asterisk end of the connection.

One of the problems that might occur is that Asterisk keeps on reporting a
> Got SIP response 481 "Call Leg/Transaction Does Not Exist" back from xx.xxx.xxx.xx
error.
That is caused by the fact that the VTA1000 doesn't support the message waiting indication sent out by Asterisk if you use mailbox=XXX@default (with XXX being the extension number). See also: Asterisk sip mailbox
Removing the mailbox= line made the problem go away.

Another problem might be that the VTA 1000 kept registering and de-registering every couple of seconds. Though you can place and receive calls it puts an unnecessary load on the server.
That can be resolved by using host=XXX.XXX.XXX.XX instead of host=dynamic with (XXX.XXX.XXX.XX being the IP-number of the pc the VTA-1000 is connected to).

A complete setup in sip.conf for the SIP-part of the VTA 1000 (using extension 123 and password 555) connected to a pc with IP 10.0.0.4 could look like this:

[123]
type=friend
username=123
qualify=no
secret=555
pickupgroup=
nat=yes
host=10.0.0.4
dtmfmode=rfc2833
disallow=all
allow=ulaw
context=from-internal
callerid="VTA1000" <123>
callgroup=