I am planning to subscribe on phone.com's services. Unfortunately, my connection is just 512mbps since I am just using a broadband. Do you think their services will still work well even if I have a slow connection?You will need to have a high speed (broadband) connection to use voice-over-IP.  A typical DSL connection will be rated at 600 kbps for the upload stream and 5000 kbps on the download stream.  You will NOTICE that your upload stream is almost always smaller than your download stream which becomes your limiting factor for using VoIP service. 

Really, 600 bps is more than enough. By using moderate compression, a 64bps channel is good for voice only. I have a VoIP SETUP through Vonage and one thing that I want to share with you is that with the right router or with a DD-WRT flash to your existing router if it is supported to run DD-WRT, you will want to enable QoS ( Quality of Service ) and allocate bandwidth priority for the VoIP if you may be downloading, playing online games, or streaming content at the same time as a phone call. Otherwise you will have troubles with the voice getting choppy between yourself and whomever you are chatting with.

I dropped my bandwidth to 5/1 ( 5 Megabits/sec download and 1Megabits/sec upload to save money and set my Vonage to allocate 1Mbps download and 500Kbps upload when the phone is in use, and when the phone is not in use the 1Mbps/500kbps is returned to the bandwidth for everything else on my home network.

Prior to setting up QoS I was getting choppy voice from whomever was speaking with me or delays in responses etc as well as a ticking sound complaint from the person I was talking with.

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Unfortunately, my connection is just 512mbps since I am just using a broadband