An "inbound call center" is where you go when you directly call into a call center number or where you go when you press a digit on your phone in response to a prerecorded message.
A "vertical market" includes all of the different parties involved in selling something to a customer. This might include the company making outbound calls (eg. prerecorded messages), the inbound call center, the fulfillment company, and the company that charges your credit card for services).
Not all inbound call centers are exclusive either in their vertical market or in any other party involved. In other words, you call a specific inbound call center, and they might ALWAYS be selling alarm systems from a particular company. Or you might call a different specific inbound call center, and they might ALWAYS be selling alarm systems, but perhaps from different companies. Finally, you might call yet another different specific inbound call center, and each time they might be selling something totally different.