My approach to the basic problem of the June 16, 2020 Iron Scripter Challenge. Very basic, not a function.

Convertto-T9.ps1

$Text = "EnVeLoPe"

$result = switch ($Text.ToCharArray()) {
        {$_ -in ('a','b','c')} { "2"; continue }
        {$_ -in ('d','e','f')} { "3"; continue }
        {$_ -in ('g','h','i')} { "4"; continue }
        {$_ -in ('j','k','l')} { "5"; continue }
        {$_ -in ('m','n','o')} { "6"; continue }
        {$_ -in ('p','q','r','s')} { "7"; continue }
        {$_ -in ('t','u','v')} { "8"; continue }
        {$_ -in ('w','x','y','z')} { "9"; continue }
        Default {}
    }
$result -join ""