Question about tax rebate and paying in taxes.?

We are married, filing jointly and made over $3000. We owed in about $1700 and sent that in with our return on April 15. Lines 52 and 57 on 1040 were 0, however, we did send in $1726. IRS says we get $600 back. I've tried calling the IRS to find out, they are busy to take my call now, so I'm asking here! Question is why aren't we getting full $1200? Thanks for your help.