Disclaimer- I don't own Harry Potter.
After the final battle, Harry got a cellphone. It was noted that they work well even through magic, and because he needed one for his muggle contacts that help keep wizards hidden. Hermione had one also, but Hermione preferred trying to find a wizarding alternative, and spent most of her time in places that the cellphone won't reach. Unlike Harry, who spent alot of time in muggle streets watching wizards who shouldn't be there.
Harry made a stupid mistake though, when he left it on around Arthur.
Having spent so much time with Hermione, Arthur's thirst for muggle technology only increased. And he finally knew the right questions to ask. That was the hard part.
Harry heard the first notes of his ringtone, and ran from the house, pulling out the phone. He his the button to answer the call, and started talking.
Arthur came out a half step later and started asking questions. Harry held up his hand to stave off the questions until he turned off the phone, then dropped it. "Ok, Arthur. Ask away."
"What is it?"
"It's like the telephone, only it's portable and works nearly anywhere. It has it's own battery."
"How do you charge it? Is it solar powered?"
"No, I have to hook it up to a charger every night at home."
"It runs on electricity?" Arthur had spent hours with Hermione and her parents learning everything. Eventually, he'd learned to say that right too. And telephone.
"Kind of. It has a battery that holds electricity and I have to recharge it after using it enough."
Harry listened to all sorts of other questions, before he just handed Arthur a slip of paper. "Here, Hermione's cellphone number. Feel free to call with questions."
Harry imagined he could hear the screams now. Hermione would never have peace...Not now that Arthur knew of cellphones.
A/N I thought it would be fun to introduce Arthur to Cellphones, but I couldn't figure out how to ask the questions as a hopeless wizard, so I gave Hermione the job of explaining.