When we talk about Voice over IP or All-IP, we are talking about telephony using classic network technology instead of line telephony ( PSTN ), which we used to know from analogue and ISDN connections.
All-IP describes the provision of transport technology, e.g. by the Telekom. Voice over IP, on the other hand, is the actual telephony technology that makes it possible to make calls via such an All-IP network. Since the one makes little sense without the other, it is basically unimportant which term they use.

In order to be able to make VoIP calls you need the following components:

• two VoIP clients
• a functioning network with internet connection
• a SIP-Trunk  from your All-IP provider

• a "telephone system"  IP-PBX or PBX  for short.

VoIP Clients

• VoIP clients can be telephones, i.e. hardware
• or softphones so software.

Network

• Your network should have a functioning Internet access and a configured DHCP server.
• and an NTP time server.

SIP-Trunk

• The SIP-Trunk is the equivalent to a plant connection. This means it is necessary to be able to be called by means of a telephone number and to call yourself.
  But just as with the old  PSTN technology, the SIP trunk is independent of the PBX (telephone system) used.

PBX / telephone system

• There are different providers and solutions.
  
  • Cloud solutions that run entirely on the provider's servers
  • Cloud solutions on VMs on your own server or on a server of your PBX provider.
  • On premise solutions, like in the past a small box that you can screw to the wall and connect to your own network.
• All partners we work with operate their servers within Germany. Thus they are subject to the strict data security laws of our country.
  Well-known PBX providers are e.g.
  
  • 3CX
  • Pascom
  • NFON
  • Starface

  A complete list of our PBX partners can be found here: IP-PBX

When selecting these components such as the right phone, PBX provider and SIP trunk provider (ISP), there are several things to consider.

• Selection criteria for the telephone
  • Display - size and color or b/w
  • Features (Bluetooth, USB, Gigabit Switch, Number of Sip Identities)
  • Codecs - This Decides About Voice Quality
  • Number of freely assignable  function keys (BLF)
• Selection criteria for the PBX
  • On Premise or Cloud
  • Does he support small businesses? Some providers do not deal with small customers.
  • Scope of functions (e.g.  IVR , Call Groups, BLF key assignment, etc.)
  • Support of your other software (Exchange, Outlook, Salesforce, etc.)
  • CTI / CSTA  support