Hi Shirley,
Further to what Paul said, I suggest diverting your son's calls from that number to another phone (number) by contacting the telecommunications service and having them do this remotely. The owner of the account will need to do this. Further, have your son contact his employer and get him/her to change the registered number so that he can receive future calls from clients and other people on a different number. Mitigate his losses as soon as possible and as much as possible.
Police can take a while to return property. Like Paul said, it really depends on why the police seized the phone in the first place and what they need the phone for. Did they have a court order to seize? Was this made in suspicion of an offence? If your son wants his phone back soon, I suggest speaking with a criminal lawyer (or your local community legal centre) and seeing if the police followed proper procedures before seizing your property and if not, your son will have a much stronger ground for demanding its immediate return.