Hello Gavin,
i will try to answer a part of the problem, concerning to following error mesage from Sems
Oct 13 12:36:07 jiffypop Sems[23889]: Error: while getting return code
from Ser
The message means that Ser could treat a request from Sems. It could be everything...
The best way to see what's happens, is to run Sems in debug mode: start 'sems -f your_configuration_file -E -D 3' and send me the output.
Additionnally, it would be necessary that you record the SIP messages on both interface (eth0 and loopback). I would like take a look at your Sems configuration file.
-Raphael.