Use better country code fallback in regV5.

feature/registration/src/main/java/org/signal/registration/RegistrationRepository.kt

@@ -5,9 +5,13 @@
 
 package org.signal.registration
 
+import android.Manifest
+import android.annotation.SuppressLint
 import android.app.backup.BackupManager
 import android.content.Context
+import android.content.pm.PackageManager
 import android.net.Uri
+import androidx.core.content.ContextCompat
 import com.google.android.gms.auth.api.phone.SmsRetriever
 import com.google.i18n.phonenumbers.PhoneNumberUtil
 import kotlinx.coroutines.Dispatchers
@@ -54,6 +58,7 @@ import org.signal.registration.proto.AccountData
 import org.signal.registration.proto.LinkedDeviceData
 import org.signal.registration.proto.ProvisioningData
 import org.signal.registration.proto.SvrCredential
+import org.signal.registration.screens.countrycode.CountryUtils
 import org.signal.registration.screens.localbackuprestore.LocalBackupInfo
 import org.signal.registration.screens.messagesync.LinkAndSyncProgress
 import org.signal.registration.screens.remotebackuprestore.RemoteBackupRestoreProgress
@@ -175,15 +180,41 @@ class RegistrationRepository(val context: Context, val networkController: Networ
     }
   }
 
+  /**
+   * Determines the region code to default the country picker to. In priority order:
+   * 1. The region of the device's own phone number, if the phone permission is granted and the number is readable.
+   * 2. The network operator's country.
+   * 3. The SIM's home country.
+   * 4. A best-guess region derived from the device locale.
+   * 5. US, as a last resort.
+   */
   fun getDefaultRegionCode(): String {
-    val maybeRegionCode = Util.getNetworkCountryIso(context)
-    val maybeCountryCode = PhoneNumberUtil.getInstance().getCountryCodeForRegion(maybeRegionCode)
-    return if (maybeRegionCode != null && maybeCountryCode != 0) {
-      maybeRegionCode
-    } else {
-      Log.w(TAG, "Invalid region or country code. Defaulting to US.")
-      "US"
+    return deviceNumberRegionCode()
+      ?: Util.getNetworkCountryIso(context).takeIfValidRegion()
+      ?: Util.getSimCountryIso(context).orElse(null).takeIfValidRegion()
+      ?: CountryUtils.localeToRegionCode(Locale.getDefault()).takeIfValidRegion()
+      ?: run {
+        Log.w(TAG, "No usable region from telephony or locale. Defaulting to US.")
+        "US"
+      }
+  }
+
+  @SuppressLint("MissingPermission")
+  private fun deviceNumberRegionCode(): String? {
+    val hasPhonePermission = ContextCompat.checkSelfPermission(context, Manifest.permission.READ_PHONE_STATE) == PackageManager.PERMISSION_GRANTED ||
+      ContextCompat.checkSelfPermission(context, Manifest.permission.READ_PHONE_NUMBERS) == PackageManager.PERMISSION_GRANTED
+
+    if (!hasPhonePermission) {
+      return null
     }
+
+    val deviceNumber = Util.getDeviceNumber(context).orElse(null) ?: return null
+    val phoneNumberUtil = PhoneNumberUtil.getInstance()
+    return (phoneNumberUtil.getRegionCodeForNumber(deviceNumber) ?: phoneNumberUtil.getRegionCodeForCountryCode(deviceNumber.countryCode)).takeIfValidRegion()
+  }
+
+  private fun String?.takeIfValidRegion(): String? {
+    return this?.takeIf { it.isNotEmpty() && PhoneNumberUtil.getInstance().getCountryCodeForRegion(it) != 0 }
   }
 
   suspend fun getRestoredSvrCredentials(): List<SvrCredentials> = withContext(Dispatchers.IO) {

feature/registration/src/main/java/org/signal/registration/screens/countrycode/CountryUtils.kt

@@ -1,5 +1,6 @@
 package org.signal.registration.screens.countrycode
 
+import com.google.i18n.phonenumbers.PhoneNumberUtil
 import java.util.Locale
 
 /**
@@ -7,6 +8,46 @@ import java.util.Locale
  */
 object CountryUtils {
 
+  /**
+   * Representative country for the most popular languages, used to guess a region when a locale carries no explicit
+   * country (e.g. a bare "en" or "de"). Intentionally not exhaustive; unmapped languages fall through to the caller's
+   * default.
+   */
+  private val LANGUAGE_TO_REGION: Map<String, String> = mapOf(
+    "ar" to "SA",
+    "bn" to "BD",
+    "cs" to "CZ",
+    "da" to "DK",
+    "de" to "DE",
+    "el" to "GR",
+    "en" to "US",
+    "es" to "ES",
+    "fa" to "IR",
+    "fi" to "FI",
+    "fr" to "FR",
+    "he" to "IL",
+    "hi" to "IN",
+    "hu" to "HU",
+    "id" to "ID",
+    "it" to "IT",
+    "ja" to "JP",
+    "ko" to "KR",
+    "nb" to "NO",
+    "nl" to "NL",
+    "nn" to "NO",
+    "no" to "NO",
+    "pl" to "PL",
+    "pt" to "BR",
+    "ro" to "RO",
+    "ru" to "RU",
+    "sv" to "SE",
+    "th" to "TH",
+    "tr" to "TR",
+    "uk" to "UA",
+    "vi" to "VN",
+    "zh" to "CN"
+  )
+
   @JvmStatic
   fun countryToEmoji(countryCode: String): String {
     return if (countryCode.isNotEmpty()) {
@@ -19,4 +60,18 @@ object CountryUtils {
       ""
     }
   }
+
+  /**
+   * Derives a best-guess dialing region code from a [locale]. Prefers the locale's explicit country when it maps to a
+   * real dialing region, then falls back to a representative country for the locale's language. Returns null if neither
+   * yields a usable region.
+   */
+  fun localeToRegionCode(locale: Locale): String? {
+    val country = locale.country.uppercase(Locale.US)
+    if (country.isNotEmpty() && PhoneNumberUtil.getInstance().getCountryCodeForRegion(country) != 0) {
+      return country
+    }
+
+    return LANGUAGE_TO_REGION[locale.language.lowercase(Locale.US)]
+  }
 }

feature/registration/src/test/java/org/signal/registration/screens/countrycode/CountryUtilsTest.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.registration.screens.countrycode
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import assertk.assertions.isNull
+import org.junit.Test
+import java.util.Locale
+
+class CountryUtilsTest {
+
+  @Test
+  fun `localeToRegionCode prefers explicit country when it is a real dialing region`() {
+    assertThat(CountryUtils.localeToRegionCode(Locale("en", "GB"))).isEqualTo("GB")
+    assertThat(CountryUtils.localeToRegionCode(Locale("de", "DE"))).isEqualTo("DE")
+    assertThat(CountryUtils.localeToRegionCode(Locale("pt", "BR"))).isEqualTo("BR")
+  }
+
+  @Test
+  fun `localeToRegionCode falls back to language mapping when country is absent`() {
+    assertThat(CountryUtils.localeToRegionCode(Locale("en"))).isEqualTo("US")
+    assertThat(CountryUtils.localeToRegionCode(Locale("de"))).isEqualTo("DE")
+    assertThat(CountryUtils.localeToRegionCode(Locale("fr"))).isEqualTo("FR")
+    assertThat(CountryUtils.localeToRegionCode(Locale("pt"))).isEqualTo("BR")
+  }
+
+  @Test
+  fun `localeToRegionCode falls back to language mapping when country is not a dialing region`() {
+    assertThat(CountryUtils.localeToRegionCode(Locale("en", "ZZ"))).isEqualTo("US")
+  }
+
+  @Test
+  fun `localeToRegionCode returns null when neither country nor language yields a region`() {
+    assertThat(CountryUtils.localeToRegionCode(Locale("xx"))).isNull()
+  }
+}