Most sirens have a “Radio re-broadcast” feature that is rarely used. This is a good tool, especially at night when backup is not readily available.
The radio re-broadcast feature uses your siren speaker to relay radio traffic. This achieves two things at once; it makes the suspect believe there are more officers present (because they are blinded by your lights and can’t see how many there are) and it makes sure you hear all the traffic in case of a radio failure.
Some officers don’t use this feature in an effort to keep the suspect from hearing the traffic, but they can likely hear it on your portable anyway.