How does your procurement department decide which companies it does business with?

I work in business development and admit that I do not fully understand how procurement departments choose the organizations they partner with. I understand that a decent portion is most likely based on relationships which may already be developed, but what if it is an area where there is no contact/relationship? For example, we provide expense management in areas like telecom, wireless and utilities for organizations worldwide. Many companies are constantly looking to have more accurate billing while reducing their overall spend (hard and soft costs) and that is where we come in. The area that I personally struggle with is how to even have a conversation with someone in procurement to at least have an opportunity for future interaction and the possibility to earn their business. Any feedback or assistance with this would be greatly appreciated.