asterisk users - Sep 2006 - strange problem with calls between MGCP and SIP clients(ATA's)

Hi,

We have experience problems with calls between MGCP ATA's and SIP
ATA's (Linksys PAP2-NA).
A call from MGCP or SIP to the other connects normally and the
conversation can usually last around 30 seconds and it becomes one-way
audio.

What I don't understand is how the calls can be set up and talk for a
few seconds without problems and suddenlly go wrong.  If there are
problems, such as misconfiguration, the call should not even be
connected, or at least the on-way audio problem should start right
from the beginning, shouldn't it?

I know MGCP is not very popular here, but we have quite a few of them
on hand that we would really like to use.
Any comments/suggestions are greatly appreciated.

Thanks.
Andy