If both your SPA and Xten are behind the same NAT, then using SIPPhone probably won't work so well. Only way in this case is direct.
The problem is that most softphones specifically want to "register" with a SIP proxy. The
SPA-3000 is not a SIP proxy and won't accept registrations, so registrations must be disabled on the Xten. However, you can place a call through a
SPA-3000 just the same.
You will want to turn on set the VoIP Caller Auth Method to HTTP Digest and set One Stage Dialing to Yes. Create a username and password under the HTTP Password section (all on PSTN Line). The SIP proxy on Xtel will be the
SPA-3000's IP address port 5061 (or whatever you set the SIP port on the PSTN Line to) and will use the username and password you defined in the HTTP Password section. Just remember to turn off "registration" (or something like that) in Xten or it won't work.