It usually indicates a "handler" helps by placing ads and potentially booking appointments. Some girls pay someone to do things to help them out, others have things done for them to control them. Which girls have which type of handler is sometimes hard to determine.
The fact they have different numbers and take phone calls could mean it is the good type of handler placing the ads. They are linked together because the same person placed the ads. At least that's they way I had it explained to me.