Welcome to Android Central! Which phone? What do you mean by "call theme"? Does the incoming call screen suddenly look different?
You can't get malware just by answering a spam call, so as long as you haven't been installing apps from insecure sources, or tapping on shady links from shady sites, you should be ok.
Advertising scams ("Don't hang up! This isn't a sales call! We can ...") use robodialers - machines that dial numbers for the area code and central office (the first 6 digits of your 10 digit number) starting with 0000 and going to 9999. Everyone gets called. You can't block them, you can't even complain about them now in the US (the government offices that handle scam and robodialed calls are closed.)
It has nothing to do with nothing to do with viruses, Trojans or anything else related to cellphones, just to things that have phone numbers. (Even an old "Candlestick" phone would get those calls.)
(And the "number" that shows on the callerID is usually the same as the first 6 digits as your number - so don't call it back - one of your poor neighbors is going to get your complaining call.)