My SIP telephony experience
I started using VOIP six years ago with cards. I used to make phone calls to Russia and Germany, the connection was disappointing, the voice was being distorted and I could hear constant screeching noises. I was dissatisfied with that.
A lot has changed since that time. Channels have become wider. I tried using Skype and that appeared to be pleasing but some Skype's disadvantages, however, made me realize the need of an alternative.
Skype has a number of drawbacks: 1. No alternative clients are provided in case the protocol is closed; 2. A p2p-based protocol, which means there might be outside traffic: if you pay your provider for every megabyte of the traffic, that outside traffic might turn out to be a quite a burden for your wallet; 3. Skype's services cost more than these of SIP providers for stationary and mobile phone calls; 4. All the Skype's solutions as regards integration in the existing PBX have been lacking the necessary functions and haven't been very easy-to-use, while it's not particularly convenient to splurge on a separate mobile phone or computer for Skype's sake only.
In addition to Skype, SIP is a good choice, too. SIP stands for Session Initiation Protocol. It is not a voice transmission protocol but a way to control the information transmission sessions. Type of transmitted data is specified by a separate SDP (Session Description Protocol). It works in complex with SIP. You can go to, amongst others, Wikipedia to find out more.
I haven't thought over all the hardware solutions, but they're aplenty. Software options allowing one to use SIP are just as various as the hardware ones.
All today's IP-PBX systems are SIP-compatible. Using SIP such giants as Cisco design their own versions for major corporations and intermediate firms.
I chose my provider rather quickly. So I went for UcallWeconn, as it comes up with just the service level one might expect from a company. UcallWeconn gives a common client an opportunity to prepay the services with VISA, while business clients are offered to make a contract, sign the SLA and pay the services on a bank-basis.
Another consideration in favor of this particular provider is that it copes surprisingly well with providing the services for home Internet users, which suggests that they access the Internet via a domestic router functioning as NAT.