UCM 6202 low PSTN volume


I have a UCM6202 and I have connected both FXO lines.
For internal phones I have 2 DP722 that they are sharing a common base DP752.
The problems is with the external calls using the FXO, the persons on the other end most of the times (not always) can hear me with low volume.
I have made ACIM detection that brings me 600Ω (Greece) on both lines and also PSTN detection that I keep the setting that the UCM proposes me.
I have to increase th tx gain up to +8 so the call is acceptable
When I plug a normal phone on the line the volume is great.
The analog lines that comes from the provider are through an Oxygen voip broadband router.

Do you have any thoughts what may be wrong?