let us consider following graph
we should answer on the following question
In the following figure, it is possible to measure both a positive and a negative value of $t_1$ and then calculate the corresponding phase shifts. Which phase shift is within the range $-π < Ø < π ?$ Verify that the two phase shifts differ by 2π.
now from this graph we should determine period,graph it is repeating it's phase at $0.005$ that means that period is distance between them or $0.025$,
so frequency is $1/T=40$ now for phase we should find $2*\pi*40*0.005+\phi=0$ or $\phi=-0.4*pi$
there is another value which will be
$2*\pi*40*-0.02+\phi=0$ or $\phi=1.6$ so difference between them is $2*\pi$ am i right?