For Asterisk users connecting to the PSTN, echo with the voice communication is possible and can be managed effectively with echo cancellation solutions 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:

VPMOCT256

• Up to 256 channels
• Compatible with the following Digital card:
  • TE820F (bundled as TE820BF)

VPMOCT128

• Up to 128 channels
• Compatible with the following Digital cards:
  • TE405PF (bundled as TE407PF)
  • TE410PF (bundled as TE412PF)
  • TE420F (bundled as TE420BF)

VPMOCT064

• Up to 64 channels
• Compatible with the following Digital cards:
  • TE205PF (bundled as TE207PF)
  • TE210PF (bundled as TE212PF)
  • TE220F (bundled as TE220BF)

VPMOCT032

• Up to 32 channels
• Compatible with the following Analog cards:
  • 4-port TDM410
  • 4-port AEX410
  • 8-port TDM800P
  • 8-port AEX800
  • 24-port TDM2400P
  • 24-port AEX2400
• Compatible with the following Digital cards:
  • 1-port TE121
  • 1-port TE122