Its a shame this has happened and I feel for you.
However you have said the you dont know the person you think took your delivery and that you have text messages to prove this(if you dont know him how come you have messages to and from him)
If there is no proof they actually took it it is your word against theirs which is never good.
Unfortunately, and this may not be what you want to hear, the parcel was signed for at your address. The courier rightly or wrongly left it with someone other than you but then the assumption is that anyone in your house is known to you and trusted by you.
O2 and the courier have fulfilled their contractual obligations to you.
Your only course is to report as burglary/theft get a crime reference number give the Police this person's; whom you dont know but have been messaging; telephone number and follow the O2 guidelines for reporting your phone lost or stolen.
Other than that you are stuck I'm affraid as I'm guessing neither you or your landlord has insurance that covers the communal areas of your house?
Not that this is much use to you now but I always use click and collect from my local O2 store. No problems then as only you or someone you nominate can collect.
Hope you get something sorted