It depends on the existing available methods of communication. If "the poor" have internet access, email, Twitter etc then adding phones doesn't help much. On the other hand if the poor don't have any internet access then just having a cell phone that lets you communicate with business partners across town could make a substantial difference. Imagine a simple business where one guy sells vegetables in a marketplace in an urban area and his business partner buys vegetables from farmers 10 miles away in a rural area. If the two can communicates via phone then they can coordinate buying, selling and transporting the vegetables (or any physical good) easily. They could even optimize purchases on a given day based on the "real-time" demand the one guy sees in the marketplace. In general the local economy could be optimized in ways that are a net win for everyone. Lack of phones and communication could hinder this.