The phase is not an absolute quantity, it has to be referred to another signal or to a fixed time vector, that mean you know where the time t=0 is
However you can find the amplitede since it hasn't affected by the phase: if x[n] is a time vector, we can write it as xn(t), the you can find the maximum of x[n], this will be A ==> A=max{xn(t)}
In case you have the time vector, simply take note of the time position (in seconds) for which you found the maximum (as said the cos have to start from 0 seconds in you vector). Let's suppose it is t1, since we know the frequency:
Period=1/frequency
1 Period = 2•Π rad
then
phase(rad) = 2•Π•t1/Period
In you case:
frequency=2•Π/15 ==> Period = 15/(2•Π)
phase(rad) = 4•Π²•t1/15