so much crap advice in one thread...
there is nothing to be gained by responding to the voicemail at this stage. it would be sensible to do a lot more research first.
ask your son to write down exactly what happened, in as much detail as possible - who did and said what, in what order - and exactly who else was there, and the history of interactions with them. did he get all of his books/ possessions back? are any of them damaged?
ask his friend to do the same.
ask them both for more info before you decide how to handle this.
do they all go to the same school?
have any of these people filmed similar incidents before?
what happened to that footage?
can you find any of it online?
was this particular incident uploaded/ livestreamed before the phone was unfortunately dropped?
does the school know about any of the previous incidents?
has their attitude been towards incidents of bullying?
was anyone else around when this happened, besides your son's friend?
any uninvolved members of the public?
is there CCTV in the park?
you said this girl hit your son. (something previous posters seem to have ignored. hitting someone is a criminal offence, 'assault', and usually considered much more serious than criminal damage to a telephone) - how many times did she hit him? what with? on which part of his body? is he injured/ bruised at all? if so, take photos, and consider taking him to the doctors so you have 'evidence'.
what is the school's bullying policy? and protocol for dealing with this kind of behaviour? is there a named person responsible for 'safeguarding'/ 'child protection' who you can contact to share your concerns about this?
he's only 13 years old, so could ring Childline for a chat as well if he wants to talk to someone who's not you, just for some advice from an impartial, anonymous adult.
you could call the NSPCC helpline if you wanted to talk to someone and discuss the different options for dealing with this kind of problem.