Build any PBX. In the language you already use.

<?php // PHP
$call = json_decode(file_get_contents('php://input'), true);
$vip  = $call['caller']['number'] === '+421905000000';
echo json_encode($vip
  ? ['format' => 'dotsip.route-result', 'format_version' => 1, 'action' => 'transfer', 'phone' => 'phone-1']
    : ['format' => 'dotsip.route-result', 'format_version' => 1, 'action' => 'answer', 'agent' => 'recepcia',
     'prompt' => 'Volajúci je nový zákazník. Ponúkni mu termín konzultácie.']);

# Python (Flask)
@app.post("/route")
def route():
    call = request.get_json()
    if call["caller"]["number"] == "+421905000000":
        return {"format": "dotsip.route-result", "format_version": 1, "action": "transfer", "phone": "phone-1"}
    return {"format": "dotsip.route-result", "format_version": 1, "action": "answer", "agent": "recepcia",
            "prompt": "Volajúci je nový zákazník. Ponúkni mu termín konzultácie."}

// Node.js (Express)
app.post("/route", (req, res) => {
  const vip = req.body.caller.number === "+421905000000";
  res.json(vip
    ? { format: "dotsip.route-result", format_version: 1, action: "transfer", phone: "phone-1" }
    : { format: "dotsip.route-result", format_version: 1, action: "answer", agent: "recepcia",
        prompt: "Volajúci je nový zákazník. Ponúkni mu termín konzultácie." });
});

// C# (.NET minimal API)
app.MapPost("/route", (Call call) => call.Caller.Number == "+421905000000"
    ? Results.Json(new { format = "dotsip.route-result", format_version = 1, action = "transfer", phone = "phone-1" })
    : Results.Json(new { format = "dotsip.route-result", format_version = 1, action = "answer", agent = "recepcia",
                         prompt = "Volajúci je nový zákazník. Ponúkni mu termín konzultácie." }));