You never specified why making it ring is a problem. Does it not have a plug on the end?
I know you probably have a digital phone system, but as someone else mentioned, if there’s a fax machine or modem in the office, plug the phone into that jack and call the number.
The official term is electrofishing and I believe it is usually illegal without special research permits. You often hear this technique used to catch catfish; I found a few pictures from the 1950’s of guys hold very large catfish that the telephoned.
I believe you drive a rotary phone with a car/marine battery, run the tip and ring leads in the water, and then dial the phone. I am not sure where the battery connects to the phone and that is probably just as well. Supposedly adjusting the frequency will affect different sizes and types of fish. I tried to find a good link to verify my understanding, but alas, my foogle gu is not up-to-snuff.
As an aside, I read about electrofishing in an outstanding novella, Poachers by Tom Franklin. It is easily one of the best stories I have ever read.
If you don’t want to build a circuit, here is an LS100 on eBay for the freakishly low price of $25. Of course, you’d need to wire the phone up with an RJ11 jack and if you do that, you may as well take Bildo’s suggestion and plug it in to the fax line.