I cannot get the 3102 to work either.
If I plug in my VOIP Provider settings directly to the 3102 it will send faxes all day long quite happily.
But if i configure it as an extension in 3cx then it never works. The reason being is that 3cx keeps forwarding the t38 invite from the internal ip of the 3102 when i look at the wireshark.
In the wireshark i see the call from the 3102, to 3cx to our voip provider and the call rings the other end and gets answered. I then see that the 3102 says it is a fax and starts a t38 session. 3cx then passes the reinvite to the voip provider but shows the internal lan ip which the voip provider acknowledges to 3cx and the rest cannot complete as the internal lan cannot be accessed by the voip provider.
The document about setting it up cannot work and is missing a lot of settings - it can only work internally.
I raised a support ticket but was told that sending faxes via 3cx is not supported (which was new to me) so we couldnt progress.
I need some answers but will have to find them myself. I have played with some NAT settings and some other settings and am getting close. I know i can figure it out. It will be to do with sending the external ip via nat etc through 3cx, but there are so many settings and so little documentation from Linksys about what each does and when and why you would use it.
There are about 7 - 10 settings that I can play with, but that gives a lot of combinations and a lot of faxing and wireshark captures/analysis for each. But we are getting there
As soon as I find the solution I will post it.