UrlHelper does not support tel:

Problem/Motivation

As detected by #2484693: Telephone Link field formatter InvalidArgumentException with 5 digits or fewer in the number, PHP does not guarantee the parse_url function results for URIs.
The problem is that PHP's parse_url is 'designed' for url's not for URIs.
Because by documentation parse_url:

This function is intended specifically for the purpose of parsing URLs and not URIs.

That can lead to many issues as this function is widely used in the Core as shown in the following table:

The ones which work:

File/Class	Function/Method	Result
core/includes/batch.inc	_batch_finished	OK
core/includes/install.core.inc	install_check_translations	OK
core/includes/install.core.inc	install_retrieve_file	OK
\Drupal\Component\Utility\UrlHelper	externalIsLocal	OK
\Drupal\Component\Utility\UrlHelper	parse	OK
\Drupal\Core\DrupalKernel	initializeRequestGlobals	OK
\Drupal\Core\Database\Database	convertDbUrlToConnectionInfo	OK
\Drupal\Core\Utility\UnroutedUrlAssembler	assemble	OK
\Drupal\language\Form\NegotiationUrlForm	validateForm	OK
\Drupal\language\Plugin \LanguageNegotiation\LanguageNegotiationUrl	getLangcode	OK
\Drupal\language\Tests \LanguageUILanguageNegotiationTest	testLanguageDomain	OK
\Drupal\language\Tests\LanguageUrlRewritingTest	testDomainNameNegotiationPort	OK
\Drupal\link\Plugin\Field\FieldWidget\LinkWidget	getUriAsDisplayableString	OK
\Drupal\menu_link_content\Entity\MenuLinkContent	preSave	OK
\Drupal\node\Tests\PagePreviewTest	testPagePreview	OK
\Drupal\search\Tests\SearchLanguageTest	testLanguages	OK
\Drupal\simpletest\BrowserTestBase	getAbsoluteUrl	OK
\Drupal\simpletest\BrowserTestBase	setUp	OK
\Drupal\simpletest\WebTestBase	getAbsoluteUrl	OK
\Drupal\system\Tests\Form\RebuildTest	testPreserveFormActionAfterAJAX	OK
\Drupal\system\Tests\Pager\PagerTest	testActiveClass	OK
\Drupal\system\Tests\Pager\PagerTest	testPagerQueryParametersAndCacheContext	OK
core/modules/update/update.manager.inc	update_manager_file_get	OK
\Drupal\views\Plugin\views\display\PathPluginBase	validatePath	OK
\Drupal\views\Plugin\views\field\FieldPluginBase	renderAsLink	OK
\Drupal\views_ui\ViewEditForm	getDisplayDetails	OK

Not OK:

\Drupal\Core\Url	fromUri	Not OK (see the issue summary)
\Drupal\Core\Validation\Plugin\Validation \Constraint\PrimitiveTypeConstraintValidator	validate	Not OK
\Drupal\link\Plugin\Field\FieldWidget\LinkWidget	getUserEnteredStringAsUri	Not OK (it transforms tel:xxx to internal:tel:xxx)
\Drupal\link\Plugin\Field\FieldWidget\LinkWidget	validateUriElement	Not OK (partly because of getUserEnteredStringAsUri)
\Drupal\link\Plugin\Validation \Constraint\LinkExternalProtocolsConstraintValidator	validate	Not OK (for both tel:911 [error], tel:65536 [error] and tel:0123456789 [failure])
\Drupal\menu_link_content \Plugin\migrate\process\d6\InternalUri	transform	Not OK (converts the tel:xxx URL to internal:/tel:xxx)
\Drupal\shortcut\Controller \ShortcutSetController	addShortcutLinkInline	Not OK (converts the tel:xxx URL to internal:/tel:xxx but it's not likely to happens in shortcuts)
core/modules/system/system.module	system_retrieve_file	Can throw notices but not likely to be used with a tel URI