This idea appeals to me on many levels. Obviously I'd want a phone in my casket just in case I pop back to life. Although I'm fairly certain that if I wake up in a casket I would be able to scream through six feet of dirt, I'd still want a phone as a backup.
You might argue that the embalming fluid would eliminate any chance of going back to work on Monday. And the fact I specified cremation in my will reduces the odds of waking up in a casket. But I am cautious by nature when it comes to issues such as being buried alive. There is a non-zero chance my last wishes are misinterpreted then the undertaker runs out of embalming fluid and decides to fake it, and my purported death is nothing but an extra good nap. It could happen, and I want a phone. I also want an outlet and a charger. And I want my casket to be at least 2,000 square feet with 10-foot ceilings, ventilation, good lighting, and a bathroom. So yes, I get the whole cell phone thing.
I'm sure my readers realize this trend creates an opportunity for some world class pranks. Begin by pretending you are putting the deceased person's cell phone in the casket and then pocket it. Later you can call the relatives of the departed and ask questions such as "How's the weather out there?" You might accuse the living of being too hasty about dividing up your possessions. Tell them you have befriended some worms but you are concerned they are up to no good. If they ask how you are doing, say everything was fine until you farted. There's no end to the comedic opportunities.