John hasn't bothered with a mobile phone in years, so it feels odd, initially: the weight of Harry's cast-off in his pocket. Sleek and futuristic, it's light years away from the clunky plastic Nokia he had before Afghanistan, and every time he takes it out and slides up the screen, it feels like he's in a pretentiously low-key phone advert.
To begin with, it doesn't see a lot of action. Texts and missed calls from Harry, of course - Harry, who seems desperate to keep in touch, even though John can't cope with her at the moment (struggling to cope with himself); Ella, confirming his appointments (or, God knows, making sure he hasn't topped himself); occasional Army administrators setting up his pension or sorting out his medical treatment. He even gets the occasional call from friends, though each conversation feels disconnected from reality, like John's just playing a part, and he rings off as soon as possible.
And then John Watson meets Sherlock Holmes, and his phone is being used to summon serial killers. Which is strange.
Next, of course, he moves in, and ends up in what seems like an incredibly bad sitcom (He's a high-functioning sociopath! He's a wounded ex-Army doctor! Together they fight crime!), and his phone is filled with endless messages from Sherlock. How long would a 180lb man take to bleed to death at 28 degrees with 79% humidity? is a fairly typical request. Why is everyone so stupid? can be expected every time he works on a case; also Come to flat immediately!, usually followed by Now! and, if Sherlock is particularly excited, If you don't get here in ten minutes, hundreds of people could die!. More often, it's Buy bread and Buy nicer bread and Where did Mrs H put my laundry? I have no socks.
But at least that's normal, for a phone. Messages for him. The thing is, John's phone is apparently no longer just his. Sherlock uses him as his own personal texting machine; John goes along with it when it's summoning serial killers, and even when it's sending pithy little comments to Lestrade's phone ("He keeps trying to block my number," Sherlock complains, almost surprised), but balks when Sherlock can't be arsed to text in a takeaway order himself.
Also, this sort of thing happens with increasing frequency:
It's bright and chilly and John's walking down Charing Cross Road, looking in book shop windows. His phone beeps. It's Lestrade - Have u seen him? No, John thinks, he hasn't, which is as well, because Sherlock was sawing away on his bloody violin at two o'clock that morning, and if John sees his flatmate, he might punch him in the face. Sorry, not today, he texts back.
He's sitting in St Paul's, head tilted back, staring at the architectural enormity of the place, wondering if he ever believed in a god, when his phone rings. It's Mrs Hudson, alarmed and voluble, and he stands up and shoulders his way past the tourists, making for the exit and trying to understand what she's banging on about and why Sherlock would have tried to dangle out of their living room window.
He's in the V&A bar with Clara, oppressed by William Morris tiles, tears (her), sympathy (him) and she's got a problem (both of them). The phone rings. "Come and get your psychopath out of here!" someone shouts. "I'm warning you, Watson, if you d-- OH MY GOD, WHAT ARE YOU DOING? STOP THAT!" John grins, suddenly, involuntarily. "Anderson?" He shrugs helplessly at Clara, fishes out a tenner, kisses her on the cheek, and leaves (gratefully). "Anderson, is that you? What's he doing?"
And very occasionally:
The weather's warmer, and he cuts through Regent's Park from the flat to Camden where a recently discharged colleague has moved in with her boyfriend. He sighs as his phone beeps, and checks the message. How stands the wax on Icarus' wings? It's at this point that John does contemplate throwing the arsing thing in the duck pond and/or suggesting that Sherlock's brother get a therapist himself. He's fine, he texts back. We had cocopops for breakfast. Your code is ridiculous.
More than once (a lot more than once) John takes Harry's old phone out of his pocket, taps it thoughtfully in his palm, and considers tossing it in the bin. He never does. Ho goes home, where Sherlock can inevitably be found haranguing Mrs Hudson, or engaging in a scientific experiment with last week's milk, or lying on the sofa, hands pressed together, a supplicant at the altar of logic, and says, "Why can't people just phone you?" And Sherlock will come out with some unfeasibly brilliant insight, or (alternatively) say that he dropped his phone in a sewer whilst following a suspect, or (once), "Rationally, alien intelligence must exist," at which point Why can't people just phone you? becomes a pretty redundant question, and John sighs, feels a bit exasperated and (depending on how exasperated) a bit affectionate, and plugs in the phone charger.