10.1.4.2. Call Between Two Access Points 
Figure 169. Signaling for an access point to another access point call
Signaling from and to the connected telephones is always transmitted
over the IP network to the call processing infrastructure in the OpenScape
4000 central system, and from there back again.
Figure 170. Voice link (payload) for an access point to another access point call
The central call processing infrastructure establishes the voice link
and makes all features available. In this case, the voice link between
the two access points is switched in the IP network with the aid of the
HG 3575 modules. Each of the access points occupies one channel of the
available transport capacity to the IP network.