There is nothing magical about a computer "virus". They are called "viruses" because, once they are run on the "host" computer, they will
replicate and send out copies of themselves.
I can COPY viruses all day long. Thousands and Thousands, and the PC will not be infected. I can transmit those viruses via regular dial-up modem,
or cable modem, or DSL, and the receiving computers will not be affected. In order for a computer to become "infected", someone (or some software)
must "run" the program.
(The following explanation isn't really correct, more of an analogy, but I didn't feel like writing a dissertation tonight, and besides, I could see
your eyes starting to glaze over from here!
Whenever one computer talks to another computer, it goes through a 7 stage process twice, with one being in reverse. That 7 layer process is often
called by computer geeks the "7 layers of the OSI model". The bottom two are Data-link and physical. Almost all (land line) phone circuits operate
at the physical level, with a smattering of data-link level.
With that having been said, phone lines WERE "hacked" years ago. It was called "phone phreaking". Remember the movie "War Games", where
Matthew Broadrick used a pull-tab to short a payphone to trick it into thinking he had deposited money? That was REAL back in the day. There were
other things you could do. In the Anarchist's Cookbook, it tells a way to take out an entire area's phone system.
Besides, the phone network has been around over 100 years. The "Internet" as it is today less than 15. (Yes, the true "internet" has been around
since the mid 60s, but let's not get Darpa in the mix.
We are in "computers and networking" where telephones were in, let's say, 1910. Do you think phones in 1910 were as reliable as they are now?