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Who is the cheapest SIP trunk service? For USA...

There are more than 200K SIP trunk provider around the world, you will need to start comparing prices

There are more than 200K SIP trunk provider around the world, you will need to start comparing prices

Also, some carriers have something called "Tiered-pricing", so calling one area might be cheaper than a "Flat-rate" carrier, but calling other areas might be significantly more expensive. Also, some flat-rate carriers may not be able to provide you with 100% coverage, especially to those expensive areas.