Ear Piece Sound Cut

[bigdog101]

I'm not sure if this is just SIP thing, but when I talk on my Aastra 57i phone, the sound from the connected party stops. I know this may happen on speaker phones, but usually doesn't happen if I'm on the handset. Is there a configuration to change this? I just want to make sure if my customer tries to interrupt me, I can hear them.

Here is my equipment.

PBX (PiaF)
Digium TDM2400P (Analog with echo cancellation)
Aastra Phone 57i

I am on a very low usage Ethernet network. Calls are made thru the SIP to the PBX and the bridged to the pots lines thru the TDM2400 analog card.

Any Ideas?

Tx,
Dave