There are still phone booths? Maybe he was leaving it for Clark Kent.
Actually, it was a phone room. In the old federal office buildings, the press and others would leave a meeting and go to the phone room to make calls.
Also, the laptop was left with other things, and it was staged so that it would be discovered. Also again, it is not proven it was Imran Awan left it. The things left were Awan’s but maybe the things were left by another.
It was previously a phone booth, but the phones had been removed, so really it was more like a closet.