For anyone that gets the " could not fill input buffer " error message, try using fxotune -i xxxxxxx (where xxxxxxx is a phone number that will ring and no one will answer). Every time I tried the fxotune -i 4 (or any other number besides 4, it gave me the " could not fill input buffer " error message). Google had all of 5 results when searching for fxotune AND "could not fill input buffer", and 3 of those were copies of the script that produced that error message.
As I've always said, the guys that program this stuff are computer geniuses, but seem to lack the ability to document what they've done. It is Linux. It is free. You get what you pay for, so don't complain.