This week we upgraded an aging but capable iPhone 6s to an iPhone 12 mini. (Location: Marco Island FL). Call quality is markedly worse on the 12 mini, with sound distortion and drop outs (literally sitting in the exact same spot from which the 6s worked perfectly days earlier.)
Local Verizon store rep says the local 5g tower is being worked on and will be for another 2-3 months! Tried forcing the phone to use LTE, but it made no difference; rep said phone has 5g SIM card so can't be made to use LTE.
As a last resort, I forced the phone to do wifi calling (Airplane Mode On + Wifi On + Wifi Calling On). Calls became perfectly stable and clear, but of course that's a poor work around.
Come Monday our plan is to return the 12 Mini and get 6s reactivated, unless there is a good fix.
Sorry to hear about your signal issues. The SIM Card for the iPhone 12 mini is both 4G and 5G, and you can keep it on 4G service. Here is how:
iPhones can be set to LTE only as per the instructions listed by NoahS. The sim needs to be 5G capable to use 5G but it will also work on LTE only also.