This can be done but it involves perl and javascript to do.
The first step is to use unverified perl to extract the existing phone numbers from the existing respondents data in the database. The second step will be to send that information out as javascript verification. Step 1 will also remove the extra characters that sometimes appear in phone numbers because exact matching is hard with phone numbers. What I mean by this is if you have 012 345 6789 entered by a respondent, then another respondent could try to enter 0 12 345-6789. These are essentially the same number just formatted differently. So we remove the extra non numeric characters so the matches will be does 0123456789 equal 0123456789. This is known as fuzzy matching because its not exactly what the respondents entered but its really how we should compare the numbers to check for duplicates. We will do this in the perl so the javascript can execute verification rather quickly.

In the post verification javascript of the question put the following unverified perl. You will need to tweak things in the code for your specific domain.


var entered_phone_number = SSI_GetValue(\"myphonenumber\");
var existing_phone_numbers = 
[%
Begin Unverified Perl

## Step 1
## Connect to the database
my $dbh = DBI->connect(\"dbi:mysql:mydatabasename:mydatabasehost\", \"mydatabaseuser\", \"mydatabasepassword\");

## Retrieve all existing phone numbers
my $sth = $dbh->prepare(\"SELECT `myphonenumberfield` FROM `datatable_containing_myphonenumberfield` WHERE `myphonenumberfield` IS NOT NULL AND `myphonenumberfield` <> ''\");
$sth->execute();

## You could just output directly as javascript here instead of storing the values in an array first
my @phonenumbers = ();
while (my $row = $sth->fetchrow_arrayref())
{
    my $ph_num = $row->[0];
    # Remove all non digit characters
    $ph_num =~ s/\\D+//go;
    push(@phonenumbers, $ph_num);
}

$sth->finish;
$dbh->disconnect;

## Step 2
## Output as javascript for verification

my $output = \"{\";
my $sep = \"\";

foreach my $ph_num (@phonenumbers)
{
    $output .= $sep;
    $output .= '\"'. $ph_num .'\": 1';
    $sep = \",\";
}

$output .= \"};\";
return $output;

End Unverified 
%]

if (entered_phone_number in existing_phone_numbers) //phone number exists/already taken
{
    strErrorMessage = \"Please enter a different phone number\";
}

Hope this helps in getting you started.

Can not duplicate phone number of respondent with other respondents