In general inphonex service is ok for the price they offer with World Unlimited calls, especially with the SIP credentials. Howerver, for me I found the call quality is more important than the flexibility and found it is bit lower than vonage call quality. You might be able to save almost $10 per month compared to vonage price and services. I have not tried the telephonic customer service, but their online customer service is excellent. They are very fast to respond and give accurate information all the time.
I would recommend iphonex for those who don't mind about call quality and bit delay in dialing etc. but to save some bucks a month.