The following configuration has not been tested for PSTN failover. The FXS port is registering to a sipXecs 3sipXcom 14.10.3 PBX2 PBX and handles inbound/outbound fax transmissions as well as voice.
The first page as seen below shows the firmware and hardware version that this unit was running.
Click on Admin Login and Advanced and then click on the SIP tab under the Voice settings. The following page will be displayed:
Set RTP Packet Size to 0.020 and then click on the Submit All Changes button at the bottom of the page.
Dial plans are tricky in the SPA3102. The following dial plan is an attempt to use 9 to dial through the PBX and no 9 to dial through an attached PSTN line. Just before the end of the page set the Dial Plan to:( [1-8]*xx| 9, xxxxxxx | 9, <:1>[2-9]xxxxxxxxx | 9, 1 [2-9] xxxxxxxxx | 9, 1 900 xxxxxxx !| 9, 011xxxxxx. | 111|0|00|[2-9]xxxxxxxxx<:@gw0> xxxxxx|1xxx[2-9]xxxxx.<:@gw0> | 0<:@gw0> | 11<:@gw0> xxxxxxS0|xxxxxxxxxxxx.)
Click on 'Submit All Changes' when done with this page.
All of these settings can be seen in the following three screen shots:
If your DNS is configured properly the gateway should register and be able to make calls at this point.
I think that is all I changed from the defaults, and these settings work on both of my PAP2's. For some reason this does not work consistent with all the Linksys/Sipura models, for example I ended up using different settings on a SPA 2100.