Are there other unknown numbers for a cell phone In GTA Online?

[Miles (Tails) Prower]

Like 911, which you need to dial? Just wondering...

[Klobby]

try putting phone numbers from single player into online and see it for yourself

[krashd]

I think once you get a high end apartment you are supposed to be able to call strippers, makes me wonder if you can call the hitchhiker girls from the storymode.

[PixelDonut]

Originally posted by krashd:

I think once you get a high end apartment you are supposed to be able to call strippers, makes me wonder if you can call the hitchhiker girls from the storymode.

You can't do the last part, you can call strippers however.
But you'll need to 'flirt' with them in the stripclub (that minigame where the guard can't see you touching her) and fill up that bar.

@OP:

Check this link
http://gta.wikia.com/wiki/Phone_Numbers