Is Rick delusional or isn't he?
1. He had to go looking for the phone when it rang, he clearly didn't notice it before hand.
2. Why would his mind make a phone ring? Wouldn't (in his mind) he think that phones and such no longer work? Wouldn't he just have a vision of Laurie instead?
3. Why would he imagine a woman’s voice he doesn't know and on a clear phone line have a conversation with her and then tell him they will call back in 2 hours, then a call back in 2 hours with a man’s voice he doesn't know on a clear phone line and then have Laurie talk to him on a weird sounding crackly phone line?
What if the people calling Rick are in fact prison staff and they are calling different areas of the prison to see if anyone will answer the phone because they hear shooting and such going on?
A Doctor/ nurse, a guard, a warden or other staff?
What if they had a camera or the phone line was open and they saw/heard Rick and Herschel talking to know his name?
What if they found Laurie and brought her to a different infirmary?
We don’t know the prison layout and neither do our heroes, maybe Carl found some sort of sub-infirmary and thought it was the real or only one, would a little kid know if in fact he was in the right place?
What if it was one of them watching Carol?
I don’t want Laurie Un-killed either but this is T.V. we are talking about here, anything is possible and the things I've mentioned are feasible.