I need to find a VOIP that has the clearest sound. I am aware of services like Vonage and Skype. I want to know from REAL LIFE users which service is truly the best. I have a wireless internet connection and I will be calling the United Kingdom. Thanks to all 
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Most VoIP providers use the same technology.
The VoIP provider only has control over one key factor of your call that determines voice quality: - Codecs.
Most VoIP providers use G.711u or PCMU Codec for highest voice quality. This is the standard high bandwidth codecs in use for interfacing with PSTN telephone networks and provides equal or better quality than standard PSTN based phone systems.
Some VoIP providers may default your account to a low-bandwidth Codec such as G.729a - which is fully acceptable, but uses less than half the bandwidth of G.711u/PCMU. Some times it is prudent to use low bandwidth Codecs. The loss in voice "fidelity" reduces probablility of voice quality problems caused by data bottle necking.
Other factors that your VoIP provider doesn't have control over inlcudes your Internet speed, your Internet connection quality, Internet data packet delays and packet jitter, or data bottle-necking to your local network, including QOS. Enabling QOS is vitally important on your local network as it gives highest priority to VoIP packets when competing with email, browsing, and file downloads, etc.
Most full service VoIP providers will supply you with VoIP hardware that is pre-configured (and locked) for their service only. If they supply you with a Voip ATA (analog telephone adapter) with built-in Router, then it will typically have QOS enabled for you. But, if you put your VoIP adapter behind your own pre-existing NAT-Router, then you may have problems unless you know how to set your Router for SIP and RTP Port Forwarding to function properly with VoIP.
I buy and configure my own unlocked VoIP adapters and configure them to work with BYOD (bring your own device) VoIP providers like CallCentric, Les.net, InPhonex, etc. I like the Linksys SPA2102-NA as it has built-in Router and QOS. When connected directly to the Internet Modem (no external router), then it provides excellent VoIP call quality.