$curlOptions = array(
    CURLOPT_SSL_VERIFYPEER => false,
    CURLOPT_SSL_VERIFYHOST => false,
    CURLOPT_VERBOSE => 0
);

$squid = $this->container->getParameter("squid"); //parameters set to contain proxy informations

$proxyHost = $squid["host"];
$proxyPort = $squid["port"];
$proxyUser = $squid["login"];
$proxyPassword = $squid["password"];

$proxyAddr = $proxyHost;
if($proxyPort != "") {
    $proxyAddr.=":".$proxyPort;
}

$userPasswordString = $proxyUser.":".$proxyPassword;

$curlOptions[CURLOPT_PROXY] = $proxyAddr;
$curlOptions[CURLOPT_PROXYUSERPWD] = $userPasswordString;

$httpClient = new CurlClient($curlOptions);
$client = new Client($this->clientSid, $this->authToken, $this->clientSid, null, $httpClient);

// ...

foreach ($client->incomingPhoneNumbers->read() as $phoneNumber) {
    var_dump($phoneNumber->phoneNumber);
}