The police has no agreement with the telcos, in the contrary, IMSI catchers ('Stingrays') are a convenient way for lawful interception without involvement of telcos. From a telco's perspective, IMSI catchers kind of hack their network. In any case, you can build your own IMSI catcher, German hackers – among others – published instructions already years ago.
Do you have a source for that? Basically, IMSI catching does not require any cooperation with telcos. On the other hand, it is easy to imagine that some telcos might betray their customers – especially under the cover of secrecy.
