For the Poisson distribution, m=0.4 calls/minute. In 12 hours (9am to 9pm) the number of calls averages at 720*0.4=288. The average number of calls per 15 minutes is 15*0.4=6. Po(m;x)=e^-6 * 6^x/x!=0.9999. We need x.
y=6^x/x!=403.39 approx. Let's make a table
x | y
3 | 36
4 | 54
5 | 64.8
6 | 64.8
7 | 55.5
9 | 27.8
We can see from this simple table that x=5 or 6 calls per quarter hour yields the best match.
If we change the time period to 30 minutes, m=12 and y=e^-12 * 12^x/x! and 12^x/x!=162754 approx.
x | y
10 | 17062
11 | 18613
12 | 18613
13 | 17182
x=11 or 12 is the peak. For m=24 calls an hour, x peaks at 23 and 24. So there's a pattern and the average number of calls a minute is around 0.392 calls at best. This is to be expected.