VoIP telephony does not have to work in complete isolation from “traditional” telephone lines and traditional PSTN, ISDN or GSM access. Devices used for making telephone calls over the Internet may also be equipped with ports enabling connection of ordinary analogue telephone line, digital ISDN connection or be equipped with a slot enabling insertion of a GSM SIM card.
In the case of devices such as VoIP gateway, router with VoIP gateway or VoIP phone, they can be equipped with a telephone port FXO type, to which it is possible to connect a traditional analog line PSTN, through which we feed the phone signal to the device. Most often it is one telephone port. Thanks to this solution, the user has the ability to receive incoming calls also from a telephone number assigned to a traditional telephone line attached to the device, as well as the ability to choose whether he wants to call via the Internet using VoIP telephony or call via a regular telephone line and PSTN network. It should be added here that most often devices equipped with an FXO port have the ability to configure the device in such a way that it automatically calls emergency numbers (or other numbers indicated by the user) via a PSTN line. This functionality is set in the device call rules or in the “Dial plan” part of the device.
Another solution can be VoIP gateways equipped only with FXO ports, to which we can connect four, eight or more analog lines. What are these devices used for? They are used in conjunction with VoIP PBXs (IPPBX PBXs) to enable users of such a system to communicate with each other using analog telephone lines. Devices of this type can also be used to transfer telephone line signals to other locations in connection with ordinary VoIP gateways with FXS ports, about which you can read here (the need to transfer internal numbers of a traditional PABX PBX to another location via IP network).
Some more advanced VoIP gateways have S0 (BRA) type ports to which it is possible to connect the access line of digital fixed-line telephony ISDN 2B+D (two call channels) or E1 (PRA) type ports to which we can connect ISDN 30B+D (30 call channels).
Through such ports in VoIP devices, we are able to integrate VoIP telephony with ISDN digital telephony and enable system users to make phone calls in both technologies. In VoIP gateways of this type, the possibility of creating rules for incoming and outgoing calls is available, so that they can be better integrated, for example, with PABX telephone exchanges connected to ISDN accesses. Thanks to this, it is possible to use a model which in such devices is the most popular, so that incoming calls are made through ISDN numbering, while outgoing calls are made through VoIP access, and in emergency cases only through ISDN access. Such a configuration allows to reduce the cost of outgoing calls and maintain the existing numbering based on the ISDN technology.
Some VoIP gateways are also equipped with slots enabling insertion of SIM cards of any mobile network operator and making calls also in this technology. Such devices are a combination of VoIP gateway and GSM gateway solutions into one integrated whole. Using this type of solution, as in the case of devices with ISDN ports, it is possible to enable users to make outgoing calls through one of the telephone media (VoIP and/or GSM) or to create appropriate rules for making telephone calls.
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