diff --git a/cmd/server/main.go b/cmd/server/main.go
index c2fd709..fc87266 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -50,6 +50,8 @@ func run() error {
 		return fmt.Errorf("webhook handler: %w", err)
 	}
 
+	adminHandler := ghl.NewAdminHandler(cfg.InboundAPIKey, s)
+
 	r := chi.NewRouter()
 	r.Use(middleware.RequestID)
 	r.Use(middleware.RealIP)
@@ -61,6 +63,7 @@ func run() error {
 	r.Get("/oauth-callback", oauthHandler.HandleCallback)
 	r.Post("/api/ghl/v1/webhook/messages", webhookHandler.HandleWebhook)
 	r.Post("/api/ghl/v1/webhook/uninstall", webhookHandler.HandleUninstall)
+	r.Put("/api/admin/locations/{locationId}/sender-id", adminHandler.HandleSetSenderID)
 
 	srv := &http.Server{
 		Addr:              ":" + cfg.Port,
diff --git a/internal/cast/client.go b/internal/cast/client.go
index 786a86d..b7ddbec 100644
--- a/internal/cast/client.go
+++ b/internal/cast/client.go
@@ -29,9 +29,13 @@ func NewClient(baseURL, apiKey, senderID string) *Client {
 	}
 }
 
-func (c *Client) SendSMS(ctx context.Context, to, message string) (*SendResponse, error) {
+// SendSMS sends an SMS via Cast API. senderID overrides the client-level default when non-empty.
+func (c *Client) SendSMS(ctx context.Context, to, message, senderID string) (*SendResponse, error) {
 	req := SendRequest{To: to, Message: message}
-	if c.senderID != "" {
+	switch {
+	case senderID != "":
+		req.SenderID = senderID
+	case c.senderID != "":
 		req.SenderID = c.senderID
 	}
 
diff --git a/internal/cast/client_test.go b/internal/cast/client_test.go
index d4d605c..eddacc2 100644
--- a/internal/cast/client_test.go
+++ b/internal/cast/client_test.go
@@ -32,7 +32,7 @@ func TestSendSMS_Success(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "")
-	resp, err := client.SendSMS(context.Background(), "09171234567", "test message")
+	resp, err := client.SendSMS(context.Background(), "09171234567", "test message", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -49,7 +49,7 @@ func TestSendSMS_APIError(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "")
-	_, err := client.SendSMS(context.Background(), "09171234567", "test")
+	_, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err == nil {
 		t.Fatal("expected error, got nil")
 	}
@@ -70,7 +70,7 @@ func TestSendSMS_SuccessFalseInBody(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "")
-	_, err := client.SendSMS(context.Background(), "09171234567", "test")
+	_, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err == nil {
 		t.Fatal("expected error, got nil")
 	}
@@ -96,7 +96,7 @@ func TestSendSMS_WithSenderID(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "CAST")
-	_, err := client.SendSMS(context.Background(), "09171234567", "test")
+	_, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -115,7 +115,7 @@ func TestSendSMS_WithoutSenderID(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "")
-	_, err := client.SendSMS(context.Background(), "09171234567", "test")
+	_, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -130,7 +130,7 @@ func TestSendSMS_Unauthorized(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_badkey", "")
-	_, err := client.SendSMS(context.Background(), "09171234567", "test")
+	_, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err == nil {
 		t.Fatal("expected error for 401, got nil")
 	}
@@ -158,7 +158,7 @@ func TestSendSMS_RetryOn429(t *testing.T) {
 	defer srv.Close()
 
 	client := NewClient(srv.URL, "cast_testkey", "")
-	resp, err := client.SendSMS(context.Background(), "09171234567", "test")
+	resp, err := client.SendSMS(context.Background(), "09171234567", "test", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
diff --git a/internal/ghl/admin.go b/internal/ghl/admin.go
new file mode 100644
index 0000000..4e79279
--- /dev/null
+++ b/internal/ghl/admin.go
@@ -0,0 +1,78 @@
+package ghl
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+)
+
+// LocationConfigStore is the store interface used by AdminHandler.
+type LocationConfigStore interface {
+	UpdateSenderID(ctx context.Context, locationID, senderID string) error
+}
+
+// AdminHandler exposes internal management endpoints.
+// All routes require a valid Authorization: Bearer <adminKey> header.
+type AdminHandler struct {
+	adminKey string
+	store    LocationConfigStore
+}
+
+func NewAdminHandler(adminKey string, store LocationConfigStore) *AdminHandler {
+	return &AdminHandler{adminKey: adminKey, store: store}
+}
+
+// HandleSetSenderID sets or clears the per-location Cast sender ID.
+//
+//	PUT /api/admin/locations/{locationId}/sender-id
+//	Authorization: Bearer <INBOUND_API_KEY>
+//	{"sender_id":"CAST"}
+func (h *AdminHandler) HandleSetSenderID(w http.ResponseWriter, r *http.Request) {
+	if h.adminKey == "" || r.Header.Get("Authorization") != "Bearer "+h.adminKey {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	locationID := chi.URLParam(r, "locationId")
+	if locationID == "" {
+		http.Error(w, "missing locationId", http.StatusBadRequest)
+		return
+	}
+
+	body, err := io.ReadAll(io.LimitReader(r.Body, 1024))
+	if err != nil {
+		http.Error(w, "failed to read body", http.StatusBadRequest)
+		return
+	}
+
+	var payload struct {
+		SenderID string `json:"sender_id"`
+	}
+	if err := json.Unmarshal(body, &payload); err != nil {
+		http.Error(w, "invalid JSON", http.StatusBadRequest)
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
+	defer cancel()
+
+	if err := h.store.UpdateSenderID(ctx, locationID, payload.SenderID); err != nil {
+		slog.Error("admin: failed to update sender_id", "location_id", locationID, "err", err)
+		if err.Error() == "location not found" {
+			http.Error(w, "location not found", http.StatusNotFound)
+			return
+		}
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+
+	slog.Info("admin: sender_id updated", "location_id", locationID, "sender_id", payload.SenderID)
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte(`{"ok":true}`))
+}
diff --git a/internal/ghl/oauth.go b/internal/ghl/oauth.go
index 6f5fd0b..555fe54 100644
--- a/internal/ghl/oauth.go
+++ b/internal/ghl/oauth.go
@@ -28,6 +28,7 @@ type TokenStore interface {
 	SaveToken(ctx context.Context, record *store.TokenRecord) error
 	GetToken(ctx context.Context, locationID string) (*store.TokenRecord, error)
 	UpdateToken(ctx context.Context, locationID, accessToken, refreshToken string, expiresAt time.Time) error
+	UpdateSenderID(ctx context.Context, locationID, senderID string) error
 	DeleteToken(ctx context.Context, locationID string) error
 }
 
diff --git a/internal/ghl/oauth_test.go b/internal/ghl/oauth_test.go
index 5db217c..c96586f 100644
--- a/internal/ghl/oauth_test.go
+++ b/internal/ghl/oauth_test.go
@@ -201,6 +201,13 @@ func (m *inMemStore) UpdateToken(_ context.Context, locationID, accessToken, ref
 	return nil
 }
 
+func (m *inMemStore) UpdateSenderID(_ context.Context, locationID, senderID string) error {
+	if m.token != nil && m.token.LocationID == locationID {
+		m.token.SenderID = senderID
+	}
+	return nil
+}
+
 func (m *inMemStore) DeleteToken(_ context.Context, locationID string) error {
 	if m.token != nil && m.token.LocationID == locationID {
 		m.token = nil
diff --git a/internal/ghl/webhook.go b/internal/ghl/webhook.go
index 4d7a82d..cf92f8f 100644
--- a/internal/ghl/webhook.go
+++ b/internal/ghl/webhook.go
@@ -138,8 +138,14 @@ func (h *WebhookHandler) processOutbound(webhook OutboundMessageWebhook) {
 		return
 	}
 
-	slog.Info("webhook: calling cast api", "message_id", webhook.MessageID, "to", localPhone)
-	_, err = h.castClient.SendSMS(ctx, localPhone, webhook.Message)
+	// Look up per-location sender ID; fall back to Cast client default if unset.
+	var senderID string
+	if rec, err := h.store.GetToken(ctx, webhook.LocationID); err == nil && rec != nil {
+		senderID = rec.SenderID
+	}
+
+	slog.Info("webhook: calling cast api", "message_id", webhook.MessageID, "to", localPhone, "sender_id", senderID)
+	_, err = h.castClient.SendSMS(ctx, localPhone, webhook.Message, senderID)
 	if err != nil {
 		slog.Error("webhook: cast send failed", "message_id", webhook.MessageID, "to", localPhone, "err", err)
 		h.updateStatus(ctx, webhook, "failed")
diff --git a/internal/store/mongo.go b/internal/store/mongo.go
index 5966b23..d363a88 100644
--- a/internal/store/mongo.go
+++ b/internal/store/mongo.go
@@ -18,6 +18,7 @@ type TokenRecord struct {
 	ExpiresAt    time.Time `bson:"expires_at"`
 	InstalledAt  time.Time `bson:"installed_at"`
 	UpdatedAt    time.Time `bson:"updated_at"`
+	SenderID     string    `bson:"sender_id,omitempty"` // per-location Cast sender ID; overrides global default
 }
 
 type Store struct {
@@ -83,6 +84,22 @@ func (s *Store) UpdateToken(ctx context.Context, locationID, accessToken, refres
 	return err
 }
 
+func (s *Store) UpdateSenderID(ctx context.Context, locationID, senderID string) error {
+	filter := bson.D{{Key: "location_id", Value: locationID}}
+	update := bson.D{{Key: "$set", Value: bson.D{
+		{Key: "sender_id", Value: senderID},
+		{Key: "updated_at", Value: time.Now()},
+	}}}
+	res, err := s.collection.UpdateOne(ctx, filter, update)
+	if err != nil {
+		return err
+	}
+	if res.MatchedCount == 0 {
+		return errors.New("location not found")
+	}
+	return nil
+}
+
 func (s *Store) DeleteToken(ctx context.Context, locationID string) error {
 	filter := bson.D{{Key: "location_id", Value: locationID}}
 	_, err := s.collection.DeleteOne(ctx, filter)