You have no option other than pay. O2 delivered and was signed for by someone you knew. That then became your responsibility and liability.
It is now a legal matter and hopefully the police can help. O2 fulfilled their responsibility to supply and deliver to you so have every right to expect payment.
Presumably you reported it as stolen and the adviser took the necessary steps to block the phone. Take a visit to Citizens Advice Bureau for further advice.
If you have a crime reference and can prove non delivery to you personally i.e. you did not know the person who signed for it ask o2 to chase this with the courier (you only found out who the person was after the delivery not before). If not do you have any insurance that covers theft .
https://community.o2.co.uk/t5/Other-Products-and-Services/Lost-or-Stolen-O2-Device-How-to-Report-it/...
Go to CAB for advice on your options. If you say this guy is unknown to you then they may be able to advise as to negligence by the courier and ultimately O2 as they use the courier but O2 are responsible and therefore they haven't fulfilled their obligations.
I Feel the key here is the fact it was a shared area and not your own door when you get deliveries is this the norm for parcles to be left in an insecure area or do the couriers only deliver to your door etc .
You have not signed for the delivery and I would pursue this with the police as long as the phone has been barred from all networks by o2 (who will know the s/n sent) the phone is worthless to the thief etc and may well mysteriously appear .
