Over the past several months I have tried and tested many Internet Telephony Solution Providers (ITSPs). Some of them were great, others were, well, less then stellar. Over the course of my testing I devised a set of criteria that allows me to quickly determine if I will consider an ITSP.

• Low number of hops - less then 15 hops is ideal.
• Low latency - less then 100ms is ideal for jitter free VoIP calls.
• Multiple service location end points.
• If one data center is down you can reroute your calls to another location.
• Allows you to select the closet location with the fewest number of hops and the lowest latency.
  
• No monthly fees and low per minute fees.
• Maximum of 2 cents per minute. Discounts for volume are a good thing.
  
• Let's face it, there is no such thing as an "Unlimited" plan. Either you'll save money by just paying 1.2 cents per minute or the ITSP will cancel your account for excessive usage.
• Minimum of 10 concurrent call sessions (outbound). More capacity can be requested as volume grows.
• Supports inbound numbers for a low monthly fee.
• Per call outbound Caller ID - This is required for our Telephony SaaS application - InphoniteVoice.
• G729a support - Needed to compress calls without significant loss of quality.

Here's my short list of ISTPs that meet those requirements:

• Gafachi - (www.gafachi.com)
• Voip.ms - (www.voip.ms)
• Teliax - (www.teliax.com)
  

If you know of any other ITSPs that meet these requirements please let me know!