Parse the phone number withsscanf() or preg_match(), then translate with PHP's native numeric text formatter, and conditionally handle leading zeros. Demo

$usPhone = '123-456-7809';

$digits = sscanf($usPhone, '%1s%1s%1s-%1s%1s%1s-%2s%2s');

$f = new NumberFormatter('en', NumberFormatter::SPELLOUT);
echo implode(
    ' ',
    array_map(
        fn($d) => ($d !== (string)(int)$d ? 'oh ' : '')
            . (!(int)$d ? 'oh' : $f->format((int)$d)),
        $digits
    )
);

Output

one two three four five six seventy-eight oh nine

103-050-0000 becomes:
one oh three oh five oh oh oh oh oh

103-050-4050 becomes:
one oh three oh five oh forty fifty

Here is another approach using preg_replace_callback() to which substrings get translated and how. Demo

$regex = <<<'REGEX'
~
 (?:
    ([1-9]\d)(?=(?:\d{2})?$)  # 1: double-digit in last segment
    |(0)                       # 2: zero
    |([1-9])                   # 3: single digit
 )
 -?                            # consume optional delimiter
~x
REGEX;

$f = new NumberFormatter('en', NumberFormatter::SPELLOUT);

echo ltrim(
         preg_replace_callback(
             $regex,
             fn($m) => sprintf(
                 ' %s',
                 array_key_last($m) === 2
                     ? 'oh'
                     : $f->format(intval($m[0]))
             ),
             $usPhone
         )
     );

As a deviation of the original requirements to help developers with phone numbers to Australian speech, here is a version which does not respect two-digit values in the last segment, but respects, hundreds, thousands, etc. as well as multiple consecutive numbers. Demo

$regex = <<<'REGEX'
~
 (?:
    \b([1-9]0{2,})\b # 1: hundreds, thousands, etc
    |(\d)\2+         # 2: multiples
    |(0)             # 3: zero
    |([1-9])         # 4: single digit
 )
 -?                  # consume optional delimiter
~x
REGEX;

$f = new NumberFormatter('en', NumberFormatter::SPELLOUT);
$mapping = [
    2 => 'double',
    3 => 'triple',
    4 => 'quadruple',
    5 => 'quintuple',
    6 => 'sextuple',
    7 => 'septuple',
    8 => 'octuple',
    9 => 'nonuple',
    10 => 'decuple',
];

echo ltrim(
         preg_replace_callback(
             $regex,
             fn($m) => ' ' . match (array_key_last($m)) {
                 1, 4 => $f->format((int)mapping[strlen(rtrim($m[0], '-'))] . " " . (f->format((int)$m[2]) : 'oh'),
                 default => 'oh',
             },
             $usPhone
         )
     );

$usPhone = '800-000-7755' becomes eight hundred triple oh double seven double five