This is a good question.
http://wiki.mikrotik.com/wiki/NetworkPr ... of_Service
I would mark the local traffic by having a nice big address-list with all the local (country/exchange) subnets.
Then I would use a Queue Tree to manage that traffic together with the rest.
To keep the customers happy - local exchange traffic should not interfere with the Internet traffic.
To avoid buffering of all traffic at places you can't control - manage it in the Queue Tree in your router even if this means total max capacity will be lower and local exchange will not be used up to its full potential.
To get the address-lists with the subnets to mark it - use a looking glass service that can give you the subnets in text format.
Maybe someone can share their experience as well?