We’re not actually supposed to believe that there was no signal on Sherlock’s phone, right? Network irregularities aside, either most phones work in the lab or most don’t, and Sherlock would have switched networks ages ago if this were a persistent problem easily fixed. Further, he surely knew Mike’s phone-keeping habits already.
Pretending no signal is his first little manipulation of John, a deliberate attempt to extract something more from him. And that’s a quick decision being made in this gif. Notice how his eyes linger when he looks back down? So intrigued.
[I’m sure this is established already but it’s just occurred to me.]