Assume the guilty person lies (both statements), and each innocent person tells the truth.

Test P guilty (P lies; Q and R truthful):

• If P is guilty, then P’s “I did not steal” is false and “Q stole it” is false $\\rightarrow$ consistent.
• Q says “R did not steal” (true) and “I did not steal” (true) $\\rightarrow$ consistent.
• R says “I did not steal” (true) and “I do not know who did it” (can be true) $\\rightarrow$ consistent.

Test Q guilty (Q lies; P and R truthful):

• P says “Q stole it” (true if Q guilty) and “I did not steal” (true). Then Q’s two statements must both be false, so “R did not steal” would be false $\\Rightarrow$ R stole; contradiction (two thieves).

Test R guilty (R lies; P and Q truthful):

• P says “Q stole it” (would be false if R guilty), contradicting P’s truthfulness.

Only the first case is logically consistent. Hence, $\\textbf{P}$ stole the phone.