Some trainers will give you evolutionary stones (Fire Stone, Thunder Stone, Water Stone, Leaf Stone), but other than that, I don't think there are any other useful people that you can give your number to.
The trainers that give you the stones are as follows:
Fire Stone - Schoolboy Alan on Route 36
Water Stone - Fisher Tully on Route 42
Thunder Stone- Lass Dana on Route 38
Leaf Stone- Picnicker Gina on Route 34
In Crystal (not sure about HGSS) there is an easy method to get phone calls. That is by continuously turning daylight savings on and off (at your mom's house). Using this method makes it easier to get the stones to evolve pokémon like Growlithe into Arcanine.
Hope I Helped!