Peppa still uses an old phone https://www.google.com/imgres?imgurl=https%3A%2F%2Fstatic.wi... I couldn't find the exact frame in the video, but there are a few where she or a friend is using a phone like that, and also a few with a wireless phone.

Peppa's most iconic use of a telephone is clearly:

https://www.youtube.com/watch?v=jil0WCh_UoQ

Peppa uses a cordless phone (but not "mobile", it uses a short range transceiver so that you can wander around your home with it) and her mother calls the Sheep household which has a more traditional phone like the pictographs.

Probably because it's pretty obvious it's a phone, since it has a distinguishable shape and all. As silhouettes used in these signs are rather simple, I would imagine a "modern" phone would look like a million other things.

Also, do you know how telegraphs look? Have you ever used one?

You mean the phone app icon? The symbol that's in the middle of the Whatsapp logo? I think it'll be fine for a while. Probably for as long as physical emergency phones exist.

you figure, kids still know what a floppy disk represents despite the tech being obsolete for 20 years now. skeuomorphism means designs like phone handsets retain some informational value long after the actual, original use is gone.

It's still the symbol for the phone app, so even if it doesn't make sense to a young person, they likely still understand it as the phone symbol.