Help > Functions and applications > VoIP telephony
VoIP telephony
Internal and external telephony is via VoIP (exception: depending on the device variant, internal analogue telephony is also possible).
The number of available VoIP channels and therefore the number of simultaneous calls possible depends on the device variant and the codecs used, determined by the VoIP operation mode set. The overviews below give information about possible combinations.
By upgrading with a NET module (COMtrexx Business) or the installation of a second network adapter/card (COMtrexx VM), there is the possibility to operate the internal IP telephony in a network separated from an external Internet telephony.
*Further help under COMtrexx hardware variants
*Further help under COMtrexx VM variants
*Further help under Network interface (second)
COMtrexx Business: overview of available channels, codecs and modules 
Installed and configured 4FXS modules reduce the available VoIP operation modes and therefore also the maximum number of available VoIP channels. If a 4FXS module has been added at a later point of time so that the current VoIP operating mode is not possible, the VoIP operation mode has to be changed manually.
VoIP operating mode
Classic
Broadband
VoIP oriented
VoIP maximum
VoIP channels (max. possible)
54
64
66
80
codecs internal
G.711
+
+
+
+
G.729
+
–
–
–
G.722
+
+
–
–
codecs external
G.711
+
+
+
+
G.729
+
–
+
–
G.722
+
+
–
–
one 4FXS module
+
+
+
–
two to five 4FXS modules
+
–
–
–
NET module
+
+
+
+
COMtrexx Next: overview of the available channels and codecs 
VoIP operating mode
Classic
VoIP maximum
VoIP channels (max. possible)
73
80
codecs internal
G.711
+
+
G.722
+
–
codecs external
G.711
+
+
G.722
+
–
COMtrexx Advanced/VM: overview of the available channels and codecs 
VoIP operating mode
Classic
VoIP oriented
VoIP maximum
VoIP channels (max. possible)
190
370
500
codecs internal
G.711
+
+
+
G.729
+
–
–
G.722
+
–
–
codecs external
G.711
+
+
+
G.729
+
+
–
G.722
+
–
–
External Internet Telephony 
Using Internet telephony, calls and data are transferred via the Internet. This requires an account at a VoIP provider. The transmission quality and the reliability of Voice over IP depends to a great extent on the quality of the Internet connection used.
Configuration of a DNS server is necessary for translating the phone number.
*Further help under Network
Internal IP telephony 
The connected VoIP telephones receive the data needed for registration (VoIP registrar, internal number and password) by provisioning via the provisioning server of the PBX. Before configuring internal numbers and associated passwords, users have to be created.
*Further help under User
*Further help under Provisioning
External Private Branch Exchanges 
The PBX handles external extensions in the same way as internal VoIP telephones.
VoIP telephones are connected to the PBX network via a VPN tunnel.
*Note: If necessary, contact your network administrator to find out the VPN settings.
*Further help under VPN
*Important: When configuring an external extension, remember that these telephones can only make emergency calls if specific requirements have been met.
*Further help under Emergency calls
*Important: When setting up an external extension, please note that the availability of an NTP server must also be guaranteed for these telephones.
*Further help under Date and Time
Quality of Service (QOS) 
The PBX uses DiffServ to give voice packets priority and to achieve better voice quality for VoIP calls. DiffServ can be evaluated in networks that include active network components such as routers or switches, to enable packets to be forwarded according to their priority.
Requirement:
•Existing and enabled DiffServ support on all active network components
As proposed in RFC 4594, the DSCP values are set for VoIP calls (signalling (SIP): CS5, voice (RTP): EF).
*Note: DiffServ is usually not supported by VoIP providers. As a result, this function may not be taken into consideration under certain circumstances on the Internet.
Configuration
VoIP channels: 
•Configuring VoIP operation mode.
−Administration > VoIP > VoIP configuration > Presetting of the VoIP channels
•Distributing the VoIP channels.
−Administration > VoIP > VoIP configuration > Distribution of the VoIP channels
•Reserving VoIP channels for VoIP provider.
−Exchange lines > Providers and Accounts > > RTP
External Internet Telephony: 
•Configuring at least one VoIP channel as external VoIP channel.
−Administration > VoIP > VoIP configuration > Distribution of the VoIP channels
•Creating a VoIP account.
−Exchange lines > Providers and Accounts > New
•If required, importing current VoIP provider data.
−Exchange lines > Providers and Accounts > Import
•Entering account and access data for the VoIP account.
−Exchange lines > Providers and Accounts > > Access data
•Checking registration.
−Exchange lines > Providers and Accounts > Status
•Creating a call distribution for incoming calls.
−Exchange lines > Exchange line routing > Exchange line routing incoming > Call distribution for
•If required, making VoIP settings (jitterbuffer, echo compensation) for increasing the VoIP call quality.
−Exchange lines > Providers and Accounts > > RTP
•If required, reducing the Interval NAT Keep Alive.
*Note: If no calls are possible via VoIP, although account registration and perhaps even an initial call were successful, this might have something to do with a very short timeout configured in the firewall. Reduce the Interval for NAT Keep-Alive for all used providers.
−Exchange lines > Providers and Accounts > > SIP
Internal IP telephony: 
•Configuring a VoIP channel as internal channel for each internal VoIP user planned.
−Administration > VoIP > VoIP configuration > Distribution of the VoIP channels
•If required, configuring the SIP port of the internal VoIP registrar.
−Administration > VoIP > VoIP configuration > SIP
•If required, changing the jitterbuffer value to increase the VoIP call quality.
−Telephones > Assignment >