From: stephen@sprunk.org   
      
   On 22-Jun-14 13:14, Adam H. Kerman wrote:   
   > Jishnu Mukerji  wrote:   
   >> On 6/21/2014 11:33 PM, hancock4@bbs.cpcn.com wrote:   
   >>> I have no idea what the voltage/currents are on VOIP phone   
   >>> lines.   
   >>   
   >> On my VOIP phone line which is an optical fiber, both voltage and   
   >> current are zero as far as I can tell :P   
   >>   
   >> Oh you meant from the modem to the phone? My old POTS phone from   
   >> AT&T still works so ..... :)   
   >   
   > That's hardly the test. Try using a 28.8k modem and let us know if   
   > you can get it to synch with another modem.   
      
   Most VoIP gateway vendors recommend getting a real POTS line if you   
   intend to use modems or fax machines.  Gateways are typically set up to   
   use G.729 or some variant of G.722, which are specifically tuned for   
   human speech and don't work well for other things, e.g. music or   
   modem/fax tones.   
      
   Some fancier gateways support T.38 for faxes, but in that case, the   
   local gateway pretends to be the far-end fax and decodes the data   
   itself, then transmits the data to the far gateway, which then reencodes   
   it for transmission to the remote fax.   
      
   AFAIK, there is no standard like T.38 for modems.  Why would you want to   
   use a modem over a VoIP line anyway?  If you already have IP access,   
   then just send the data via IP.   
      
   S   
      
   --   
   Stephen Sprunk         "God does not play dice."  --Albert Einstein   
   CCIE #3723         "God is an inveterate gambler, and He throws the   
   K5SSS        dice at every possible opportunity." --Stephen Hawking   
      
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