Personally I don't like this idea.

The better solution as for me is to introduce some entity (let's call it `CallHandler`) which will have a method (let's call it `handle` or whatever) which will accept a `Call` as input, look for the subscriber by their number and then apply tariff, which will be exposed through `Subscriber`'s getter.

In this case `Subscriber` will know nothing of `Call` which is good.