"Someone went missing," the caller says, "and [the alleged victim] was with the guy and now they're telling him like, 'Where's the guy?' and all that."

The caller, who ID'd himself as the alleged victim's son, claimed he was on the phone with his father when he heard two guys arguing and roughing him up.

The caller says he could hear his dad saying, "Why are you hitting me" and "stuff like that" ... adding he believed his dad was "in trouble."

8/26 10:10 AM PT -- Police say Mackensie Alexander and another man beat the hell out of a guy on Tuesday ... and cops say it all started because the men confronted the alleged victim about "an incident that had transpired the day prior."

In police documents, obtained by TMZ Sports, cops say Alexander and Evins Clement showed up to an Immokalee, FL residence to interrogate a man about a "prior incident."

FYI -- cops did NOT reveal what the alleged incident was nor did they confirm the identity of the alleged victim. But, it looks like the altercation could be connected to Alexander's father's disappearance.

In the docs, cops say Alexander and Clement tried to get the man to leave with them in their car ... but they say the man refused, "due to fear of retaliation from the prior incident."

Officers say that's when Alexander "threatened the victim that he had a firearm in his vehicle if the victim continued to refuse to go with them."

Cops say when the alleged victim continued to refuse ... they say Clement grabbed the man and held him down while Alexander punched him with a closed fist in the face repeatedly.

Officers say both Alexander and Clement then fled the scene when they heard cops' sirens approaching.

Cops add in the report the alleged victim "had a noticeable injury to his lip and forehead."

Both Clement and Alexander were eventually arrested ... with the two each being booked on misdemeanor battery charges.

In a statement released Wednesday, a spokesperson for the Collier County Sheriff's Office said, "We recognize that this is an emotional and frightening time for the family and friends of [Alexander's father] Jean Odney Alexandre."

"However, everyone must act within the law."

8/26 6:15 AM PT -- 8/26 -- Alexander was arrested shortly after leaving Bengals camp to search for his missing father ... TMZ Sports has confirmed.

Jail records show the NFL player was booked on 1 count of battery on Tuesday.

Unclear the details of the arrest -- we're working on it -- but NFL Network's Tom Pelissero reports, "My understanding is he allegedly confronted the man who left behind his father, Jean Odney Alexandre, on a berry-picking trip."