I have thoughts about this (aside from the obvious sad ones). I assume Dean had been paying Cas' phone bills all along. How long has he had one? Seems to be since he carved the sigils into D&S's ribs, and could no longer locate them on angel radar…
Cas shows up with a phone in the next episode—calls Sam, in fact, to find out where they are. Dean says, "Cell phone, Cas? Really? Since when do angels need to reach out and touch someone?"
But later, after he takes War's ring, after nearly losing Sam to the demon blood again, after Sam decides he needs to stop hunting… Dean holes up in some run-down motel, waiting for his next case, until he hears wings behind him, and Cas is inside his personal space. He's never sure why, but the angel's presence is always an even mix of comforting and terrifying—and in this case he scares the bejeezus out of him.
Cas is still on a quest for God, or Raphael in this case. Whatever—it still feels like a wild goose chase to Dean.
He agrees to help Cas, but after he turns down a bowel-seizing teleport and tells Cas they’re driving, he taps Cas’ arm on the way out the door with the back of his hand.
"Wait up," he says. "You still got that phone?"
Cas tilts his head with curiosity, but delivers the Nokia into Dean's waiting palm. It’s one of Dean’s own burner phones, he realises—Cas must’ve lifted it from the trunk. It’s an old model—shit went out of fashion a couple of years ago. He watches as Dean plugs his phone number into it and sends himself a text.
Dean hands him back the phone. "Hang onto this until I can get you a new one, okay?"
"Dean, this phone is perfectly functional. Besides, I can hear when you pray to me.”
“I know that, it’s just...it'd be nice to hear from you once in a while, okay? Tell us how the choir's going up there.” Dean steps towards the car to open the driver’s door, hoping Cas isn’t picking up on how much Dean wants to be able to contact him. Is it his fault if he worries about the guy, fighting Heaven’s Civil War? Besides, with Sam gone...the road’s pretty lonely. He’ll grab Cas a new phone and keep paying for that bill for as long as the angel is around. At least then he’ll always have someone to call.
When he glances out the window to see why Cas hasn’t got into the car, the angel is standing in place, looking down at the phone with a puzzled look on his face. There’s a tiny smile on his face as he glances quickly up and hurries to join Dean in the passenger seat.
Huh, that’s weird, Dean thinks. It’s like the dude’s never been given anything before.
