avayax,
there are many different reasons and benefits that come from SIP trunking vs. other alternatives. Price is only one of those and there are many others. The spectrum spans for customer service when you have issues to flexibility of the service, reliability and failover options, E911 flexibility for some people, and many more. If your goal is to get the cheapest cost to do 40,000 outbound minutes a month then that will result in a different set of carriers. If TW is happy with your volume and type of traffic you’re sending them, that may be a better option.
The other question I would have is to review how you have calculated your 40,000 minutes per month. I’m guessing you have a good handle on your usage and that is probably an accurate assessement, but I bring it up because I’ve seen many people try to calcuate their minutes using Asterisk CDR’s and come up with some very overstated numbers. The reasons range from simple errors like looking at the call duration vs. the billable call duration, to some of the more complex dialplan scenarios that result in very confusing ‘multiple CDRs per single call’ records that can create double, triple, … counting. The softcap minutes in SIPStation are based on the billable duration (Call answer to call hangup, 20 seconds of ringing before the call is answered is not billable but is part of the overall duration that can overstate a call if incorrect data is reviewed).