toilet fills everytime water runs in bathroom or kitchen sink?

I just replaced the toilet bowl. it is a cadet 3 by american standard. it stops after it is flushed, but then when someone turns on the kitchen or bathroom sink it starts to run for a few seconds and fill the bowl. any ideas on how to fix this or what the problem is? thanks.
I do not have a septic tank. I am attached to the town sewer.

sounds like your septic tank needs to be pumped.

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