The simplest way to send calls from IP to the PSTN it to use the Welltech FXO units. They are very cheap and do dynamic CODEC negotiation – therefore if one call is G.723 and the next is G.729 it’s no problem.
Also the units come in SIP and H.323 flavours so it opens up the number of companies you can trade traffic with.
Hope this helps, Mat Hitchcock – mhitchcock@blue-lava.co.uk