The Philadelphia Police Department said Thursday it's emergency 9-1-1 call center has recently experienced sporadic difficulties with incoming cellular-phone calls. In some cases, the police dispatcher cannot hear the caller's voice. However, the caller's cellular phone number associated with the 911 call is still displayed on the radio dispatcher's computer screen. With such calls, the police radio unit immediately contacts the callers cell phone from the listed caller ID number, according to police. Verizon, which is the police department's phone service provider, has been working with cell phone carriers to resolve this problem. Police say this problem is not caused by the department's hardware or software components.    - Inquirer staff