You don't say what kind of internet connection you have, but most likely it's due to extra noise on the line. Home phones ring because they send ~100 volts AC down the line.

When I was a kid, we had a DSL line for a while (too long!) that would disconnect whenever someone called. Very frustrating when in the middle of an online game.

Even dialup was more reliable, if slower!

Cordless phones use the same frequencies as WiFi, so they could be interfering with each other.

Or, I dunno. Is your phone using voip?

my ISP is spectrum