[Asterisk-Dev] Strange error codes

[Asterisk]

Asterisk CVS-HEAD-04/01/05-06:32:03, running on TE410P

We make around 3000 calls a day on our EuroISDN-30 PRI. During the past 
couple of days, we are encountering a whole load of "88" errors when 
trying to make a call. I've looked at the ISDN reason codes, and found 
the following:

88 58 Incompatible Destination.
The number being dialed is not capable of the type of call.
1. Calling a restricted line in unrestricted mode.
2. Calling a pots phone using unrestricted mode.
Indicates that the equipment sending this cause has received
a request to establish a call which has low layer compatibility, high layer
compatibility, or other compatibility attributes which cannot be
accommodated.

What does this mean exactly :) Is it our systems, or the Telco that has 
the problem ?

Julian.