Your call can be used to bind you almost anywhere within the city. (outside of your home, at the bank, in front of the lift etc). However, your CoK cannot bind you to T-Steppes (The Karanas) or any other combat zone.
The up side to all this is you will come to find that with all the public transportation available in Norrath now, getting around is much easier than it used to be (in EQLive).
Between the "insta-boats, the birds in Ant, Commonlands and T-Steppes, the carpets in Maj'Dul, the cloud transport in the OverRhelm, the horses in FayDwer and the Sokokors in Kunark, you can easily get out to wherever you need to go without too much time spent and the trip is always one way since you have a call for your ride home! And remember that with the exception of the 40s charge for taking a boat from Qyenos/Freeport to T-Steppes, all travel is free!
The one hour reset timer on your call is to keep it from being abused.
NOTE: In addition Druids get a variety of ports (as in EQLive) that will take them and their group to various druid rings making the trip even shorter!