asterisk users - Jun 2003 - TDMxx weirdness with 2-line SBC portable phone

This is a really strange one, IMO.

I just bought an SBC 900Mhz two-line portable phone.  My intent was to 
use it with one of my TDM20 lines with asterisk, and have the other line 
be a PSTN line.

But it appears as if for some reason I can't intermingle the two types 
of lines.  If I hook *both* lines from the TDM20 to the unit it works 
just fine.  If I hook *both* of my PSTN lines to the phone, it also 
works just fine.

But any combination of one of one kind and one of the other bollixes up 
the phone--in that case one line always rings just fine, but as soon as 
the ring voltage goes up on the other one, the display on the portable 
says, "Error" and the unit doesn't ring.  Trying to "pick
up" the line
that shows the error immediately disconnects the call.

Anyone have any idea what could be the problem here?

Thx.

B.