I ended up removing the Nicor 18888 chime from both the downstairs and upstairs. Then i purchased a new transformer from Home Depot. I think it was this one - https://www.homedepot.com/p/Hampton-Bay-16VAC-30VA-Transformer-Compatible-with-All-Video-Door-Bells-HB-130-03/309792479 and replaced the downstairs chime with the new transformer.
Next, I disconnected the doorbell wires and connected the power wires to the new transformer. Basically I bypassed the entire Nicor doorbell system.
Initially I had an issue with the transformer. I purchased one from Amazon and it didn’t work. Then after numerous attempts, I got a new transformer from Home Depot, installed it and it worked.
Regarding the chimes, well you can use your phone or you can purchase the external plugin chimes. We opted for the plugin chimes. They seem to work fine. Let me know if this helps or if you’d like me to upload a few pics.