diff --git a/tests/tools/test_send_message_tool.py b/tests/tools/test_send_message_tool.py
index 3d9da96ae..626179de1 100644
--- a/tests/tools/test_send_message_tool.py
+++ b/tests/tools/test_send_message_tool.py
@@ -770,6 +770,46 @@ class TestParseTargetRefMatrix:
         assert is_explicit is False
 
 
+class TestParseTargetRefE164:
+    """_parse_target_ref accepts E.164 phone numbers for phone-based platforms."""
+
+    def test_signal_e164_preserves_plus_prefix(self):
+        """signal:+E164 is explicit and preserves the leading '+' for signal-cli."""
+        chat_id, thread_id, is_explicit = _parse_target_ref("signal", "+41791234567")
+        assert chat_id == "+41791234567"
+        assert thread_id is None
+        assert is_explicit is True
+
+    def test_sms_e164_is_explicit(self):
+        chat_id, _, is_explicit = _parse_target_ref("sms", "+15551234567")
+        assert chat_id == "+15551234567"
+        assert is_explicit is True
+
+    def test_whatsapp_e164_is_explicit(self):
+        chat_id, _, is_explicit = _parse_target_ref("whatsapp", "+15551234567")
+        assert chat_id == "+15551234567"
+        assert is_explicit is True
+
+    def test_signal_bare_digits_still_work(self):
+        """Bare digit strings continue to match the generic numeric branch."""
+        chat_id, _, is_explicit = _parse_target_ref("signal", "15551234567")
+        assert chat_id == "15551234567"
+        assert is_explicit is True
+
+    def test_signal_invalid_e164_rejected(self):
+        """Too-short, too-long, and non-numeric E.164 strings are not explicit."""
+        assert _parse_target_ref("signal", "+123")[2] is False
+        assert _parse_target_ref("signal", "+1234567890123456")[2] is False
+        assert _parse_target_ref("signal", "+12abc4567890")[2] is False
+        assert _parse_target_ref("signal", "+")[2] is False
+
+    def test_e164_prefix_only_matches_phone_platforms(self):
+        """'+' prefix must NOT be treated as explicit for non-phone platforms."""
+        assert _parse_target_ref("telegram", "+15551234567")[2] is False
+        assert _parse_target_ref("discord", "+15551234567")[2] is False
+        assert _parse_target_ref("matrix", "+15551234567")[2] is False
+
+
 class TestSendDiscordThreadId:
     """_send_discord uses thread_id when provided."""
 
diff --git a/tools/send_message_tool.py b/tools/send_message_tool.py
index eef267368..534426607 100644
--- a/tools/send_message_tool.py
+++ b/tools/send_message_tool.py
@@ -23,6 +23,13 @@ _FEISHU_TARGET_RE = re.compile(r"^\s*((?:oc|ou|on|chat|open)_[-A-Za-z0-9]+)(?::(
 _WEIXIN_TARGET_RE = re.compile(r"^\s*((?:wxid|gh|v\d+|wm|wb)_[A-Za-z0-9_-]+|[A-Za-z0-9._-]+@chatroom|filehelper)\s*$")
 # Discord snowflake IDs are numeric, same regex pattern as Telegram topic targets.
 _NUMERIC_TOPIC_RE = _TELEGRAM_TOPIC_TARGET_RE
+# Platforms that address recipients by phone number and accept E.164 format
+# (with a leading '+'). Without this, "+15551234567" fails the isdigit() check
+# below and falls through to channel-name resolution, which has no way to
+# resolve a raw phone number. Keeping the '+' preserves the E.164 form that
+# downstream adapters (signal, etc.) expect.
+_PHONE_PLATFORMS = frozenset({"signal", "sms", "whatsapp"})
+_E164_TARGET_RE = re.compile(r"^\s*\+(\d{7,15})\s*$")
 _IMAGE_EXTS = {".jpg", ".jpeg", ".png", ".webp", ".gif"}
 _VIDEO_EXTS = {".mp4", ".mov", ".avi", ".mkv", ".3gp"}
 _AUDIO_EXTS = {".ogg", ".opus", ".mp3", ".wav", ".m4a"}
@@ -317,6 +324,12 @@ def _parse_target_ref(platform_name: str, target_ref: str):
         match = _WEIXIN_TARGET_RE.fullmatch(target_ref)
         if match:
             return match.group(1), None, True
+    if platform_name in _PHONE_PLATFORMS:
+        match = _E164_TARGET_RE.fullmatch(target_ref)
+        if match:
+            # Preserve the leading '+' — signal-cli and sms/whatsapp adapters
+            # expect E.164 format for direct recipients.
+            return target_ref.strip(), None, True
     if target_ref.lstrip("-").isdigit():
         return target_ref, None, True
     # Matrix room IDs (start with !) and user IDs (start with @) are explicit