I don't understand... w-why do you want me to say my name?
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 below indicates that it is a pre-paid 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!
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 cell phone 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.
Dean calls Castiel to the St. James Medical Clinic Center basement to identify the Enochian marks Sam has discovered on two victims' hearts; Castiel subsequently teleports to Dean's location as soon as Dean reveals it to him without hanging up.
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 which it is shown he didn't understand what he was doing while setting up the message people get. After finally getting Sam's messages, which he calls annoying, Castiel comes to Blue Earth, Minnesota, drunk, to help.
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 cell phone, suggesting that Castiel may have lost the phone when he was forced to banish himself in 5.18 Point of No Return.
Castiel calls Dean to tell him that the "trials" Metatron had him perform to seal angels into Heaven were actually part of a spell to seal them all out of Heaven. He explains that Metatron has stolen his grace for the spell. Although Castiel wants to come help Sam (who is dying), Dean tells him to go to the Bunker instead to avoid vengeful angels. Castiel does validate Ezekiel over the phone as a trustworthy angel to help Sam, unaware that Ezekiel is dead and the angel in question is actually Gadreel posing as Ezekiel. At this point, Castiel apparently no longer has a cell phone as Dean does not try to simply call him when looking for him and Castiel needs to ask a trucker for his phone and later use a pay phone to call Dean.
After learning of four mysterious disappearances in Rexford, Idaho, Castiel calls Dean from his cell phone to let him know. After learning that Nora' daughter is sick, Castiel calls her to let her know, but ends up having to leave a voicemail, which he seems to have mastered finally.
After finding the Horn of Gabriel, Castiel calls Sam and Dean and uses his phone to take a picture of the symbol and send it to them, once again showing a greater mastery of a cell phone than he had previously.