Five hundred begins it, five hundred ends it,
Five in the middle is seen;
First of all letters, the first of all figures,
Take up their stations between.
Join all together, and then you will bring
Before you the name of an eminent king.

who's the king

There was a death on Treebark Ln. The victim was identified as Mark Oswalt, who recently was married. The police went to the crime scene and they reported the death as a suicide.

Later that day, after the police left, a private detective, hired by the victim's friend who thought it was a murder, searched the crime scene and found a note the police missed.

It read,

"4,3: 8,1:_: 9,1: 2,1: 7,4:_: 6,1:9,3:_: 9,1: 4,3: 3,3: 3,2: !"

The detective took out his cell phone and started dialing the police to tell them about his findings. Once the detective opened the phone to dial, he immediately screamed out, "I SOLVED IT!"

Who was the murderer and how did the detective find out?

need a hint

Mark Oswalt used his cell phone for business calls many times a day.

ok

The note said, "It was my wife!"

The detective looked at the keypad on his phone and saw the letters next to the numbers.

2,1 would be A
2,2 would be B
9,4 would be Z