If you are using X-Ten Lite you will not be able to receive calls from some VOIP providers. ( I wish this was mentioned in the various online FAQ's , especially as people are signing up for a service and EXPECTING it to work ).  The problem is the Codec support.

Making calls is generally easy as many codecs are supported ( g711, gsm, iLBC ) , but some providers inist on using G729a when sending the call to you.  It will not negotiate, and X-Ten Lite does not have G729a support.  ( Hey guys, can it be configured to drop to iLBC or gsm ? )  X-Ten Lite has never had G729 support (I also tried an old version I had from 2004).

FYI, here is some info on some of the codecs...

G.711 ( ulaw , alaw )  64Kbps  - excellent quality, use on LAN
G.723.1  5.3 or 6.4 kbps - low quality - proprietary ( needed for H.323 )
G.729a  8kbps - Great quality - proprietary, in most commercial offerings.
GSM  13kbps - Good quality, think mobile phone...
iLBC  13.3 or 15.2 Kbps - Great (uses alot of CPU), gaining support.
Speex 2.15->22.4Kbps Variable - Great, gaining support.

I have purchased "eybeam" ( voice only ) , this is the new version of X-Ten and it has the proprietary G729a and G723 codecs ( plus heaps of others ) licensed in it.  It's about $40 AUD.

Regarding Asterisk, they cannot ship G729 and G723.1 codecs, all other codecs I mentioned are supported.  If you check your logs (/var/log/asterisk/full) you will see asterisk complaining that it can't match up the sender/receiver with a compatible codec.  

To get G729 and G723.1 support, you buy a license from digium, download them and run a registration.  See:
http://www.digium.com/index.php?menu=product_category&category=codec

You might want to read this first though....
http://www.readytechnology.co.uk/open/ipp-codecs-g729-g723.1/