kept talking to people on the phone in the basement where Lori had the baby. But the last call he received was from Lori telling him to be brave, etc. So did Rick have a mental break down after Lori died at childbirth? Was there someone actually on the other end of the line at any point?
Yes, he is having a breakdown. Hershel seems to acknowledge this with his skeptical and confused look when he picked up the phone, but has not brought it up to Rick or the others just yet.
In the show: Lori explains to Rick that the other three people he "spoke to" on the phone were Jackie, Jim and Amy - three of the others from their group who had already died.
In the comic: Lori is the only person who Rick "speaks to". In his anger/confusion when Lori identifies herself, he rips the phone from the wall but is still able to "speak to" her on the phone.