I haven't a clue to #2, but for #1, we have some suspects:
- Sally Sparrow -- She was the first person I considered while watching that scene. But I don't think she actually has the number. Also, Carry Mulligan has a bigger career now, and therefore isn't -- so I'm told -- likely to return to Doctor Who.
- River Song -- She has the Doctor's number, and the character is known to return this season. But please ... don't be River. She's worn out her welcome like the Mississippi during the rainy season. Yeah, that's a placeholder until I can think of a more clever line.
- Martha Jones -- She has the Doctor's number, and she's used it to summon him. Freema Agyeman isn't noted to return this season, though.
- Donna Noble -- Didn't she call the Doctor in the episode "Midnight"? Even if she didn't, she's likely to have his number. She just doesn't remember it. Or Who it's for. Or even why she'd call it. Never mind, she probably already cleared that number from her contacts to make room for all her other gossipy girlfriends.
- Rose Tyler -- Isn't she off in another universe? Although it looks like she's returning. I think.
- Amy Pond -- Did she leave her cellphone in the TARDIS, or did she have it with her when she got touched by an angel? And Rory probably still had his. Remember the end of "A Piece of the Action"? Now apply that to the 1930s and Earth. And Doctor Who instead of Star Trek.
- Someone in U.N.I.T. -- Brigadier Lethbridge-Stewart had a space-time telegraph unit to contact the Doctor. Maybe he updated that, and someone kept the number when they left the organization.
- Churchill's descendants -- Winston could call the Doctor ... maybe one of his great grandchildren (although aren't all grandchildren great?) might have inherited a scrap of paper including the number.
Items 7 & 8 allow for a previously unseen character. I don't think we'll see Amy and Rory again. At least I hope not, they had a fantastic conclusion to their character arc -- especially if one includes the unfilmed bit with Brian's grandson that Darvill narrated. Rose is rumored to be coming back, but she'll be along with Tennant, and they'll have their own way of barging into the story. Donna won't be back. Her character has such a sad ending, but it's a good place to leave her character in an imperfect universe. Marth and Sally aren't likely to return.
So that leaves River. I hate to say it, but that's most likely a way to bring River into one of the episodes. I really am so tired of River. I'm Rivered-out.