The stick is regulation, and the carrot is money.
Unfortunately, it's really hard to do once the govt. money is spent. I'm familiar with AU/NZ. Both are pumping significant public money into broadband infrastructure, and they took different approaches to enforcing fair network access.
In AU, they created a new company (NBN co) to run the "utility" part. See: http://whirlpool.net.au/wiki/nbn
The government is paying the monopoly incumbent to switch users onto the new network.
In NZ, our privately owned largest telco (Telecom NZ) semi-voluntarily split itself into two parts - a retail and a wholesale branch. The shareholders agreed to do this, because that meant the wholesale part (Chorus) would get a big govt. contract to lay fiber to the door.
In theory, the split is "arms length", so other network providers can access the infrastructure on fair terms.
Reference: http://www.stuff.co.nz/business/industries/5858284/Telecom-s...
It remains to be seen whether this is the right way to go about things, but both approaches do seem plausible.