It’s pretty simple, configure the Ethernet port. Configure the voice ports and a dial-peer. If you look at the UBR924 guide on CCO you’ll get it in no time. It will take about 5 minutes to read and do it. After a few times, you’ll have it down in no time.
As far as a free gateway, I did a test Call from Denver going over my cable modem (AT&T) to another CCIE lab in NYC. all I used was UBR924, and my cable modem to accomplish this. If you are trying to learn voice gateways i can’t help there.