Don't call but send mail or a fax and give them 3 working days to respond in writing to the issue. Say if they don't you will feel obliged to go to BBB. BBB will want to see what attempt you've made in writing to resolve it although noting down the calls etc you've made will be important.
If you have no mailing address for them at all - which would be unusual - ring but keep a tape record if possible of what you say. Don't record THEM because it's illegal as a rule to record without permission.
I would not pay the additional money either and it sounds like they were short on materials and didn't want to pay properly. I would be saying IF you don't return to complete the work to my satisfaction and to the level we agreed upon, I will contract someone else to do this work and remove that payment from your account. In other words if you have to pay someone else $100 you remove $100 from the first person's account. To be honest, that concrete/cement probably needs doing fairly soon as one lot will harden and unless the next people roughen the surface so the second pour will adhere, you may wind up with two sections. Darn nuisance.
But do it in writing if possible and put a limit on it. 3 days is reasonable I think..3 working days. Sometimes I will give longer depending on the issue and depending whether we've spoken about the problem before. You have so you should reduce the time frame I believe.
Sorry to hear of your woes.