IP PBX using IAX in snome VoIP Phone

I am lookin for a IP PBX solution for a small virtual s/w co with 15 SOHO staff in 10 different cities in 2 differnt countries, & not a lot of capital so the 3Com, Mitel, Ciscos
ArtiSoft Televantage etc
solutions are outa here…

So it looks like it is either
http://www.vovida.org/vocal/
or
http://www.asteriskpbx.com/main/index.html
for a PBX,

Then we were looking at these Linux based VoIP phones http://www.snomag.de
running ether the base H323/SIP or astrix claims their implemenation of IXA provide easier install bcus
signal & bearer prto (voice stream) are the in same packet & only one udp port so no problem with NAT
& higher quality due to less packet overhead compare to RIP..

Then some FXO Hardware to support the VoIP-POTS gateway in the PBX
2-4 lines ..

The real issue is Free LD & the ability to conference LD.. not so much the POTS to VoIP but the interCompany commuication …

comments on these selection or a btter way to solve the problem