Magento 2: How to call custom phtml file in another phtml file?

Replace below code with your block code

namespace Lapisbard\General\Block;
use Magento\Customer\Model\Session;

class CustomerAccount extends \Magento\Framework\View\Element\Template {
    public function __construct(\Magento\Framework\View\Element\Template\Context $context,
        Session $customerSession
    )
    {
        parent::__construct($context);
        $this->_customerSession = $customerSession;
    }
    public function isCustomerLoggedIn() {
        return $this->_customerSession->isLoggedIn();
    }
}

Then you can use it

echo $this->getLayout()->createBlock("Lapisbard\General\Block\CustomerAccount")->setTemplate("Lapisbard_General::customer_account.phtml")->toHtml();