This issue occurs using the Gigaset N300 IP Base Station with the Gigaset Dect Phones because the equipment has a limited capacity for the number of calls it can process at one time.  

It allows for only 3 consecutive calls to come in at any one time.  If any of those calls are transferred to another extension it will take up the equivalent of 2 lines on the transfer thus limited the number on incoming calls received at that time to just 1.

The issue has occurred as there was no more capacity so it will cut off the other callers to pull up more capacity to be able to open a channel to either receive in another call or to make an outgoing call.  

To overcome this you need to invest in an additional Gigaset N510 IP Base Station (this allows for 4 consecutive calls) to expand out the capacity so you are running 2 base units on your router(s).  

We would need to move over some of your team onto the new base station.  Please contact our Support Desk. and we will advise on cost to set this all up, depending on how you want it all to be configured.