Castiel's Cell Phone
Castiel: What? What? Dean--I didn't--I didn't get that.
Dean: You know it's kinda funny talking to a Messenger of God on a cell phone. It's like watching a Hell's Angel ride a moped.Castiel: This isn't funny Dean. The voice says I'm almost out of minutes!
– Castiel to Dean, 5.04 The End
Castiel first uses a phone in 4.15 Death Takes a Holiday when he masquerades as Bobby Singer. In Season 5, he uses a phone consistently, beginning in 5.02 Good God, Y'All. After engraving the Enochian Sigils into Sam and Dean's ribs at the end of 5.01 Sympathy for the Devil, he is unable to sense their location, and needs to contact them by phone to find them.
The quote above indicates that it is a pre-paid phone.
In 5.01 Sympathy for the Devil, Castiel carved Enochian Sigils into Sam and Dean's ribs. The sigils shield their location from all angels, including Castiel. To locate the brothers from this point on, Castiel relies on cellphone contact. He is the first angel, and the only one aside from Anna Milton, to resort to using human technology for this end. In 5.02 Good God, Y'All, Castiel needs to locate the Winchesters so that he can borrow Dean's amulet.
Castiel phones Dean to talk about finding the Colt. He suggests that if Dean wants to kill Lucifer, the gun may be the way. Dean agrees that they should search for it. Eager to begin looking, Castiel asks Dean's location and announces that he will be there immediately. Dean begs him to wait until morning so that he can have a few hours of sleep, then hangs up. Castiel agrees to wait and continues standing, rather forlornly, on the side of a highway.
Sam and Dean call in Castiel to help with a powerful child who is revealed to be a Cambion.
Sam calls Castiel about the pastor's daughter who claims to be a Prophet. Cas doesn't answer, so Sam leaves a message on his voicemail.
Castiel calls Dean to let him know that he is in a hospital and has lost his angelic powers. He requests money for transportation and pain medication. He also apologizes for losing faith in Dean. This call is made from a hospital phone rather than Castiel's cellphone.
"I don't understand... w-why do you want me to say my name?"
– Castiel's Voicebox, 5.17 99 Problems