It would be reassuring for all if the Police investigated false alarms and prosecuted malicious calls.
Regret to say its not that easy, If a call comes on a 999 line then we are presented with some of the callers details, however if it came via a non emergency number, which this call should have, then we get no details off the exchange equipment. So the starting point can be a problem if all the person did was utter the meassge and replace the phone.
Also, when compared to the Fire service we get a lot more calls. For example for the area I work the two fire brigages are likely to have taken around 27000calls for the year, we know that because of their reference numbers. The police will take that number of logged calls in less than a month. My force's daily totals between 1500 to 2000 a day.
Finally, its up to the CPS to decide what action will be taken. We have a person who can ring with 20 or so non malcious calls on the 999 system but they don't take action because that person has mental health problems and may need a phone in case of a real emergency. So what chance would this call have of being followed up, very little I would suggest.