The following locations and people can be called from your cellphone.

Businesses:
Bean Machine Coffee: 555-0110
Emergency Services: 911
Express Car Service: 555-2222
Express Car Service: 555-3333
Fire Proof Doors: 555-5700
Hero Shop: 555-8575
LCPD Recruitment: 555-RECRUIT
Liberty Construction: 555-1274-73
No Problemo Bail Bonds: 555-945-733-92
Storage: 555-221-3877
Superb Deli: 555-4674
The Serrated Edge: 1-800-555-5555
Used Auto Parts: 555-7300
Vinewood Laundromat: 555-5123
ZiT: 948-555-0100

People:
Blackmailer from "Call And Collect" mission: 843-555-0124
Drug dealer from "Lure" mission: 545-555-1022
Roman Bellic: 718-926-7215