PHP. Right way to do a weighted distribution

I need to distribute calls among employees. Different employees has different weights in the system. For example, one employee has to take two times more calls, than some other.

Now i'm using this scheme:

Employee 1 - rating 10.
Employee 2 - rating 9.
Employee 3 - rating 4.
Employee 4 - rating 3.

I'm using a weighted random function from this answer (Generating random results by weight in PHP?) to select a phone number to call.

Problem is, that there are not many calls a day. About 10-20. The method above would be OK on bigger numbers, but with small amount of calls, Employee 4 could easily recieve no calls on some day.

Is there a way to do weighted distribution, that works well on small numbers? Or the answer above is the best one?