asterisk users - Jul 2005 - Polycom phone echo question

Hi, all

I have Polycom SP300 phones.
Calls between those are ok and quality is great.

Then I have IAX2 soft phones (FireFly). Calls between those are OK too.

But when I have call b/w Polycom (SIP) and IAX, I have really bad echo at 
Polycom phone side. IAX phone side is OK.

Any ideas?

Thanks,
Rudolf