Just to add to this... I recently purchased an OBiLINE for use with an OBi200, and hear significant echo. I've been through every ACImpedance setting, and tried adjusting the ChannelTXGain and ChannelRXGain, but cannot get rid of it. Phones plugged into the same jack used by the OBiLINE have absolutely no echo. I can hear the echo even if I connect the OBiLINE to an OBi110's phone port (where there should be an exact 600 ohm impedance match), which completely eliminates the PSTN as a factor.
I've opened a support ticket, so I'm hoping there is some solution.
(I also get very frequent DTMF talk-off with the OBiLINE, but that's another issue...)