Any solicitor would help get your phone if you paid them. Or were you on legal aid???
If you were on legal aid do you think the public purse should really spend between 175-500 pounds per hour trying to retrieve your phone??? Errrr no!!!
I assume the phone was also some form of evidence in the case. I am afraid they have the right to keep it in case you lodge an appeal (if guilty).
If it is prima facia evidence of the offence (i.e harassment etc) they do have the power to destroy it.
As said, just write a nice letter.
Pointless writing to the Chief Constable, it will be dealt with by his secretary and be redirected straight to the Inspector of the station concerned. Therefore go straight to that Inspector in the first instance.
Also the vast majority of Police Officers will have a force email address which will be readily available from their call centres. Maybe try that means.