For Asterisk users connecting to the PSTN, echo with the voice communication is possible and can be managed effectively with echo cancellation modules from Digium. Asterisk has a range of software-based Open Source echo cancellation routines that are moderately effective in eliminating the echo that most Asterisk users experience. However, there are cases in which these algorithms are not effective. To combat this, Digium offers a line of DSP-based echo cancellation modules for all the Digium analog and digital (T1/E1/J1) cards with the VPMOCT series modules. 

The modules and the Digium telephony cards they support are as follows:

VPM064

• Up to 64 channels
• Compatible with the following Analog cards:
  • 8-port A8A
  • 8-port A8B
• Compatible with the following Digital cards:
  • 2-port TE235 (bundled as 1TE235BF)