The only surprising part is he didn't turn it off much earlier. I think he leaves it on for the entertainment value. Also because he enjoys the flirting as much as Sherlock does :D

Then he became busy with his patients.

(I hate the fics where John answers his phone/looks at his texts while consulting with a patient. Extremely unprofessional - John would never do that (no doctor would, honestly). Here I assume he was between consultations when he was texting :-)