Originally Posted by http://en.wikipedia.org/wiki/Caller_ID

With cellphones, the biggest issue appears to be in the passing of calling line ID information through the network. Cellphone companies must support interconnecting trunks to a significant number of Wireline and PSTN access carriers. In order to save money, it appears that many cellphone carriers do not purchase the North American feature Group D or PRI trunks, or SS7 trunks required to pass calling line ID information across the network.