>If a call is rejected due to this (all channels busy), will the msg "no answr from system" be displayed ?
Yes. But don't think it's your problem. You can check the saturation of the RBS using traffic observation (dbgobs tool)
Maybe there is also an interference which makes some carriers unusable.
Normally you have many timeslots for each frequency so I don't believe that there is a saturation in the DECT itself.
Maybe for the RBS - but not in the carrier frequency.
I guess you have a synchronisation issue.
How many DTM card's are managed?
How is the synchronisation management?
Incidents in incvisu?
What's the result of infocs command?
...
PS: d1.311 is R4.1.1 and d1.305 is R4.1