I don't use a case. I'll contact Sprint using email and save it as a record. Thanks for the suggestion.
BTW, I live in a metro area with good reception. According to Sprint's coverage map, my area has the "Best" coverage. On my replacement HTC One, it measures between -85dbm to -80dbm.
Follow up :
I'd like to provide an update of my issue. I have contacted Sprint, BestBuy, and HTC. The status is as follows:
Sprint rep thinks that I might got my HTC One phones from a bad batch of phone. She is going to get a Device Care Manager contact me within 24 hours.
BestBuy rep also thinks that my two HTC One phones are from a bad batch of phones. She suggested I go to another BestBuy store to exchange it.
HTC rep, on the other hand, suggested that I might want to change how I grip the phone. He thinks it is normal to have some signal variation if the antenna is covered, even after I told him the loss was up to 25dB! In his email, he asked me to provide serial number and software version on a HTC link. When I clicked the link, I only found my case was closed and I couldn't enter any information.