diff --git a/lib/Command/PhoneNumber/AddPhoneNumber.php b/lib/Command/PhoneNumber/AddPhoneNumber.php
index e65e76259a9..8785684ecb3 100644
--- a/lib/Command/PhoneNumber/AddPhoneNumber.php
+++ b/lib/Command/PhoneNumber/AddPhoneNumber.php
@@ -11,8 +11,8 @@
 use OC\Core\Command\Base;
 use OCA\Talk\Model\PhoneNumber;
 use OCA\Talk\Model\PhoneNumberMapper;
+use OCA\Talk\Service\PhoneNumberValidation;
 use OCP\AppFramework\Db\DoesNotExistException;
-use OCP\IPhoneNumberUtil;
 use OCP\IUser;
 use OCP\IUserManager;
 use Symfony\Component\Console\Input\InputArgument;
@@ -24,7 +24,7 @@ class AddPhoneNumber extends Base {
 
 	public function __construct(
 		private IUserManager $userManager,
-		private IPhoneNumberUtil $phoneNumberUtil,
+		private PhoneNumberValidation $phoneNumberValidation,
 		private PhoneNumberMapper $mapper,
 	) {
 		parent::__construct();
@@ -66,12 +66,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 		}
 		$userId = $user->getUID();
 
-		$phoneNumberStandard = preg_match('/^[0-9]{1,20}$/', $phoneNumber) ? $phoneNumber : $this->phoneNumberUtil->convertToStandardFormat($phoneNumber);
-		if ($phoneNumberStandard === null) {
+		try {
+			$phoneNumber = $this->phoneNumberValidation->validateNumber($phoneNumber);
+		} catch (\InvalidArgumentException) {
 			$output->writeln('<error>Not a valid phone number ' . $phoneNumber . '. The format is invalid.</error>');
 			return self::FAILURE;
 		}
-		$phoneNumber = $phoneNumberStandard;
 
 		try {
 			$entry = $this->mapper->findByPhoneNumber($phoneNumber);
diff --git a/lib/Command/PhoneNumber/ImportPhoneNumbers.php b/lib/Command/PhoneNumber/ImportPhoneNumbers.php
index ac9d4098426..32c46978369 100644
--- a/lib/Command/PhoneNumber/ImportPhoneNumbers.php
+++ b/lib/Command/PhoneNumber/ImportPhoneNumbers.php
@@ -11,8 +11,8 @@
 use OC\Core\Command\Base;
 use OCA\Talk\Model\PhoneNumber;
 use OCA\Talk\Model\PhoneNumberMapper;
+use OCA\Talk\Service\PhoneNumberValidation;
 use OCP\IDBConnection;
-use OCP\IPhoneNumberUtil;
 use OCP\IUser;
 use OCP\IUserManager;
 use Symfony\Component\Console\Input\InputInterface;
@@ -23,7 +23,7 @@ class ImportPhoneNumbers extends Base {
 
 	public function __construct(
 		private IUserManager $userManager,
-		private IPhoneNumberUtil $phoneNumberUtil,
+		private PhoneNumberValidation $phoneNumberValidation,
 		private PhoneNumberMapper $mapper,
 		private IDBConnection $db,
 	) {
@@ -72,12 +72,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 				continue;
 			}
 
-			$phoneNumberStandard = preg_match('/^[0-9]{1,20}$/', $row[0]) ? $row[0] : $this->phoneNumberUtil->convertToStandardFormat($row[0]);
-			if ($phoneNumberStandard === null) {
+			try {
+				$row[0] = $this->phoneNumberValidation->validateNumber($row[0]);
+			} catch (\InvalidArgumentException) {
 				$output->writeln('<error>Not a valid phone number ' . $row[0] . '. The format is invalid.</error>');
 				return self::FAILURE;
 			}
-			$row[0] = $phoneNumberStandard;
 
 			$user = $this->userManager->get($row[1]);
 			if (!$user instanceof IUser) {
diff --git a/lib/Service/PhoneNumberValidation.php b/lib/Service/PhoneNumberValidation.php
new file mode 100644
index 00000000000..01c88c2d30d
--- /dev/null
+++ b/lib/Service/PhoneNumberValidation.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Talk\Service;
+
+use OCP\IConfig;
+use OCP\IPhoneNumberUtil;
+
+class PhoneNumberValidation {
+
+	public function __construct(
+		protected IPhoneNumberUtil $phoneNumberUtil,
+		protected IConfig $config,
+	) {
+	}
+
+	/**
+	 * Validate input as a phone number
+	 *
+	 * - Local number: allow
+	 * - International number:
+	 *   1. Replace leading 00 with +
+	 *   2. Check if still valid international number
+	 *      a. If valid, strip + and allow
+	 *      b. If invalid, throw
+	 * @throws \InvalidArgumentException When the number is invalid
+	 */
+	public function validateNumber(string $phoneNumber): string {
+
+		if (
+			// Not an internation number
+			!str_starts_with($phoneNumber, '00')
+			// And matches a local number or dial-through
+			&& preg_match('/^[0-9]{1,20}$/', $phoneNumber)
+		) {
+			return $phoneNumber;
+		}
+
+		// Replace double leading zero with +
+		if (str_starts_with($phoneNumber, '00')) {
+			$phoneNumber = '+' . substr($phoneNumber, 2);
+		}
+
+		$defaultRegion = $this->config->getSystemValueString('default_phone_region') ?: null;
+		$standardPhoneNumber = $this->phoneNumberUtil->convertToStandardFormat($phoneNumber, $defaultRegion);
+
+		if ($standardPhoneNumber === null) {
+			throw new \InvalidArgumentException();
+		}
+
+		if (str_starts_with($standardPhoneNumber, '+')) {
+			return substr($standardPhoneNumber, 1);
+		}
+
+		return $standardPhoneNumber;
+	}
+}
diff --git a/lib/SetupCheck/SIPConfiguration.php b/lib/SetupCheck/SIPConfiguration.php
index 0f2fb2a387b..83cbf94b77b 100644
--- a/lib/SetupCheck/SIPConfiguration.php
+++ b/lib/SetupCheck/SIPConfiguration.php
@@ -9,6 +9,7 @@
 namespace OCA\Talk\SetupCheck;
 
 use OCA\Talk\Config;
+use OCP\IDBConnection;
 use OCP\IL10N;
 use OCP\SetupCheck\ISetupCheck;
 use OCP\SetupCheck\SetupResult;
@@ -16,6 +17,7 @@
 class SIPConfiguration implements ISetupCheck {
 	public function __construct(
 		protected readonly Config $talkConfig,
+		protected readonly IDBConnection $connection,
 		protected readonly IL10N $l,
 	) {
 	}
@@ -39,9 +41,35 @@ public function run(): SetupResult {
 		if ($this->talkConfig->getSignalingMode() === Config::SIGNALING_INTERNAL) {
 			return SetupResult::success($this->l->t('Using the SIP functionality requires a High-performance backend.'));
 		}
+
+		$query = $this->connection->getQueryBuilder();
+		$query->select('phone_number')
+			->from('talk_phone_numbers')
+			->where($query->expr()->like('phone_number', $query->createNamedParameter(
+				$this->connection->escapeLikeParameter('+') . '%'
+			)))
+			->orWhere($query->expr()->like('phone_number', $query->createNamedParameter(
+				$this->connection->escapeLikeParameter('00') . '%'
+			)));
+
+		$result = $query->executeQuery();
+		$invalidNumbers = $result->fetchFirstColumn();
+		$result->closeCursor();
+
+		if (!empty($invalidNumbers)) {
+			$message = $this->l->t("Assigned Talk phone numbers must not start with + or 00. Please remove the following numbers:\n{list}");
+			$message = str_replace(
+				'{list}',
+				implode("\n", $invalidNumbers),
+				$message
+			);
+			return SetupResult::error($message, 'https://portal.nextcloud.com/article/Nextcloud-Talk/Nextcloud-Talk-Phone/Direct-Dial-in#content-provisioning');
+		}
+
 		if ($this->talkConfig->getSIPSharedSecret() === '' && $this->talkConfig->getDialInInfo() === '') {
 			return SetupResult::info($this->l->t('No SIP backend configured'));
 		}
+
 		return SetupResult::success();
 	}
 }