... the fact that HFC-S does not have a hardware echo canceller an advantage if you decide to better use free software and maybe even improve the echo canceller.
The dahdi-hfcs is the latest Digium DAHDI kernel module svn trunk plus adopted and properly renamed zaphfc driver plus latest OSLEC echo canceller svn trunk, all ready to compile out-of-the-box. By default only the things you need to drive a HFC-S PCI A based ISDN BRI card plus some software drivers like dummy device and echo canceller