You can list the phone as a business asset using the purchase price. Some assets must be depreciated over time, but for purchases under $2500, you can usually take the purchase as an expense in year 1, rather than depreciating over time. (The trade-in is irrelevant unless it was also a business asset.)
However, you can only deduct the percentage used for business, and if audited, you need to show by some kind of contemporaneous, reliable written record, how you calculated that percentage. Contemporaneous means it is a record or calculation you performed close in time to the use of the phone, not something that you wrote down from memory the night before the audit.
