There are several competitors of Deutsche Telekom who offer leased lines throughout Germany.
The company I am working for is using leased lines from one of these providers to build a WAN between our sites in Germany. The provider is offering his service significantly cheaper than Deutsche Telekom does. However there's a downside we did not expect.
Our central site has a direct fiber connection to the provider network. At two other sites however this is not available, because the provider does not have his own network / fibre infrastructure in this city. Nevertheless he offered to connect this site to our central site.
How does he do it? He teams up with a local provider. However this local provider doesn't have a direct connection to our office in this city either - he needs to rent the "last mile" from Deutsche Telekom. This means that our WAN connection to this city is bought from one provider, but underneath its dependent on three providers.
First of all, going through three providers is of course adding several possible points of failures.
Those providers to not always talk to each other when it comes to maintenance. We've had outages that turned out to be "planned maintenance", however we were not informed and it happend in the midst of the day during our business hours.
Even worse, once we had an outage starting friday evening. Our provider did not even notice the outage. He monitors the connection to the local provider, but did not notice at all that the connection from the local provider to our office had failed. This is in spite he has an SLA that covers the end-to-end connection.
Not going with the Ex-monopolist Deutsche Telekom saves money, but it comes at additional risks.