Is there a reliable provider who allows me to customize the cart system to add phone verification?

Most of them I saw charges almost 30c for every call which I think is little steep.

You can try Maxmind Phone Verification at maxmind.com

A company I've done some work for in the past does this as well.

Have a look at http://public.ifbyphone.com/verifymenow

I believe with them you only pay for minutes (~8 cents/min) or something.