Everyone would like to speak on the computer like on the telephone and of course it would be comfortable, if this calls would be much cheaper. For this purpose the VoIP protocol is used. It is a technique for transmitting speech via the Internet or dedicated networks based on the IP protocol. VoIP is commonly called the "Internet telephony". Data is transmitted using the IP protocol, which allows us to exclude unnecessary "continuous connection". Exchange of information is not required when the caller is silent. VoIP is independent of the operator / state monopolies (freedom of choice, and potentially greater privacy), what is a big advantage. Calls inside the operator`s network are free, and the cost of calls from fixed telephony is lower than customary (specially for international calls). It provides full mobility of the user (roaming issue does not exist) and a low cost of infrastructure (compared to the traditional phone lines), as well as integration with future services such as data transfer or image. VoIP also has disadvantages, but they are few. Unfortunately, VoIP is more unreliable than the fixed telephony.
Burdensome may also be the need for a dedicated hardware or software and the Internet connection. Also hardware implementation is more expensive (I.e. apparatus) for the end user. Problematic may be also the lag due to uncontrolled full mobility and the endless possibility of the geographical location of "network termination". Conjunction with emergency numbers and other abbreviated numbers. The most common practice is to use VoIP to carry abroad calls, because the cost is many times smaller than for traditional telephony or mobile.
The various systems differ not only in the price of calls, but also in a way to "connect" with.
In the case of scratch cards connections are made via access numbers / dial-up connection (type 801, but also the usual numbers of "stationary" offered by some VoIP). In the case of IP phone, VoIP gateway or the program (typical VoIP or IM) is performed by a combination of held-Speed Internet. You can purchase the accompanying number of fixed telephony, which allows you to receive incoming calls. Currently, for this type of connection, there is the possibility of calling to any worldwide phone number (some operators restrict the ability to call for a paid hotlines and emergency calls). Additionally, some operators offer versions of the post-paid (payment after the service) except of pre-paid (paid in advance).