e and lambda and call multiplied by the interval, which is 5 minutes times 60 seconds/minute

e <- exp(1)
lambda <- 0.01 * 300

zero calls

zero_calls <- ((lambda^0)*(e^-3)) / factorial(0)

one call

one_calls <- ((lambda^1)*(e^-3)) / factorial(1)

Add the two together

ppois(1, lambda=3)
## [1] 0.1991483
zero_calls + one_calls == ppois(1, lambda=3)
## [1] TRUE