AAISP Data SIMS can be relayed on to your own L2TP Server, such as a FireBrick

FireBrick Config

<l2tp>
 <incoming name="MyL2TP" hostname="FireBrick" secret="myPassword" pppip="91.240.176.254" pppdns1="217.169.20.20" pppdns2="8.8.8.8" lcp-rate="25" lcp-timeout="240" tcp-mss-fix="true" radius="l2tp" log="ppp" log-debug="ppp">
 <match name="MySIM" graph="MySIM" calling-station-id="89442xxxxxx" remote-ip="10.0.0.4" comment="Change IP to one on your LAN"/>
</l2tp>

AAISP Config