Generally, a VoIP adapter is used to connect an analog telephone to a voice over IP service provider (VoIP provider) or VoIP PBX. VoIP phone adapters convert the analog signal of a phone into voice over IP which then uses the Ethernet (digital) connection to send the call to a VoIP provider or VoIP PBX. Device configuration is usually through a web browser on a local computer or voice commands through a connected analog telephone.
There are two types of VoIP adapters, the most popular are simply used to connect an analog telephone to a VoIP service provider or VoIP PBX. These VoIP adapters have one or two FXS(Foreign Exchange Station) ports for connecting analog telephones or fax machines to. This adapter will also have one or two Ethernet ports to connect to the Ethernet network. The other type of VoIP adapter contains FXS ports and one or two FXO(Foreign Exchange Office) ports for connecting analog telephone lines to. These are useful if you would like to have an analog fail over in case your internet connection drops out. This type of VoIP adapter will also contain one or two Ethernet ports to connect to the Ethernet network.
*Understanding FXO(Foreign Exchange Office) and FXS(Foreign Exchange Station) ports
1.FXO ports connect to the Public Switched Telephone Network (PSTN is the conventional phone line). VoIP Adapters will have either 0 or 1 FXO port. Having an FXO port on the adapter will allow you to switch between making/receiving VoIP calls over the Internet, and calls over the PSTN.
2.FXS ports connect to analogue phones or fax machines. VoIP adapters have either 1 or 2 FXS ports. 2 FXS ports for example, would allow you to have 1 cordless DECT phone and one fax machine connected to the VoIP adapter, or 2 phones each with their own VoIP line.