A tone dialer. Like this
https://images.app.goo.gl/fbdmckv44BY7fdWw9
Not for phone phreaking, just for speed-dialling.
I would make international calls frequently. I would buy calling cards. The process was: dial the 800 number on the card. Enter the id number on the card to use some of its credit. Dial the number to call. Their service would then connect me at a low rate to another country(probably making a voip call).
So I’d set up the 3 speed dial buttons with those. For each new card I’d only have to change the card’s unique number.
There an app for that now.
At least, there is on the Flipper Zero.