diff --git a/apps/api/domain/whatsapp_number.go b/apps/api/domain/whatsapp_number.go
index 7047cebf..ec310f32 100644
--- a/apps/api/domain/whatsapp_number.go
+++ b/apps/api/domain/whatsapp_number.go
@@ -10,11 +10,13 @@ func NormalizeWhatsappNumber(raw string) (string, *Error) {
 	stripped := whatsappNumberSeparators.Replace(raw)
 
 	var normalized string
+	hasExplicitCountryCode := false
 	switch {
 	case strings.HasPrefix(stripped, "0"):
 		normalized = "62" + stripped[1:]
 	case strings.HasPrefix(stripped, "+"):
 		normalized = strings.TrimPrefix(stripped, "+")
+		hasExplicitCountryCode = true
 	default:
 		normalized = stripped
 	}
@@ -33,6 +35,10 @@ func NormalizeWhatsappNumber(raw string) (string, *Error) {
 		return normalized, nil
 	}
 
+	if !hasExplicitCountryCode {
+		return "", invalidWhatsappNumberError()
+	}
+
 	if len(normalized) < 8 || len(normalized) > 15 {
 		return "", invalidWhatsappNumberError()
 	}
diff --git a/apps/api/domain/whatsapp_number_test.go b/apps/api/domain/whatsapp_number_test.go
index 70a7fa12..44d1d21b 100644
--- a/apps/api/domain/whatsapp_number_test.go
+++ b/apps/api/domain/whatsapp_number_test.go
@@ -16,6 +16,8 @@ func TestNormalizeWhatsappNumber(t *testing.T) {
 	}{
 		{name: "leading 0 becomes 62", raw: "0812-3456-7890", expected: "6281234567890"},
 		{name: "628 prefix is unchanged", raw: "6281234567890", expected: "6281234567890"},
+		{name: "62 prefix without a plus is accepted", raw: "6285335473895", expected: "6285335473895"},
+		{name: "0 prefix converts to the same number", raw: "085335473895", expected: "6285335473895"},
 		{name: "+628 drops the plus", raw: "+62 812 3456 7890", expected: "6281234567890"},
 		{name: "other country plus is dropped, kept if 8-15 digits", raw: "+6591234567", expected: "6591234567"},
 		{name: "dots and parentheses are stripped", raw: "(0812).3456.7890", expected: "6281234567890"},
@@ -33,6 +35,9 @@ func TestNormalizeWhatsappNumber(t *testing.T) {
 		{name: "non-62 number at the 15 digit ceiling is accepted", raw: "+123456789012345", expected: "123456789012345"},
 		{name: "non-62 number over 15 digits is rejected", raw: "+1234567890123456", expectsErr: true},
 		{name: "bare plus sign is rejected", raw: "+", expectsErr: true},
+		{name: "number without 0 or 62 prefix is rejected", raw: "85335473895", expectsErr: true},
+		{name: "number without prefix and with separators is rejected", raw: "853-3547-3895", expectsErr: true},
+		{name: "non-62 number without a plus is rejected", raw: "6591234567", expectsErr: true},
 	}
 
 	for _, tt := range tests {
diff --git a/libs/ui/src/domain/entities/Customer.test.ts b/libs/ui/src/domain/entities/Customer.test.ts
index 01cf4a11..d04156c7 100644
--- a/libs/ui/src/domain/entities/Customer.test.ts
+++ b/libs/ui/src/domain/entities/Customer.test.ts
@@ -11,6 +11,8 @@ describe('normalizeWhatsappNumber', () => {
   }[] = [
     { name: 'leading 0 becomes 62', raw: '0812-3456-7890', expected: '6281234567890' },
     { name: '628 prefix is unchanged', raw: '6281234567890', expected: '6281234567890' },
+    { name: '62 prefix without a plus is accepted', raw: '6285335473895', expected: '6285335473895' },
+    { name: '0 prefix converts to the same number', raw: '085335473895', expected: '6285335473895' },
     { name: '+628 drops the plus', raw: '+62 812 3456 7890', expected: '6281234567890' },
     { name: 'other country plus is dropped, kept if 8-15 digits', raw: '+6591234567', expected: '6591234567' },
     { name: 'dots and parentheses are stripped', raw: '(0812).3456.7890', expected: '6281234567890' },
@@ -28,6 +30,9 @@ describe('normalizeWhatsappNumber', () => {
     { name: 'non-62 number at the 15 digit ceiling is accepted', raw: '+123456789012345', expected: '123456789012345' },
     { name: 'non-62 number over 15 digits is rejected', raw: '+1234567890123456', expected: null },
     { name: 'bare plus sign is rejected', raw: '+', expected: null },
+    { name: 'number without 0 or 62 prefix is rejected', raw: '85335473895', expected: null },
+    { name: 'number without prefix and with separators is rejected', raw: '853-3547-3895', expected: null },
+    { name: 'non-62 number without a plus is rejected', raw: '6591234567', expected: null },
   ];
 
   it.each(cases)('$name', ({ raw, expected }) => {
@@ -40,8 +45,8 @@ describe('formatWhatsappNumberForInput', () => {
     expect(formatWhatsappNumberForInput('6281234567890')).toBe('081234567890');
   });
 
-  it('leaves a non-Indonesian number unchanged', () => {
-    expect(formatWhatsappNumberForInput('6591234567')).toBe('6591234567');
+  it('shows a non-Indonesian number with a plus so it stays valid', () => {
+    expect(formatWhatsappNumberForInput('6591234567')).toBe('+6591234567');
   });
 
   it('leaves an empty string unchanged', () => {
diff --git a/libs/ui/src/domain/entities/Customer.ts b/libs/ui/src/domain/entities/Customer.ts
index 452c1acb..57f3c37f 100644
--- a/libs/ui/src/domain/entities/Customer.ts
+++ b/libs/ui/src/domain/entities/Customer.ts
@@ -9,10 +9,12 @@ export function normalizeWhatsappNumber(raw: string): string | null {
   const stripped = raw.replace(WHATSAPP_NUMBER_SEPARATORS, '');
 
   let normalized: string;
+  let hasExplicitCountryCode = false;
   if (stripped.startsWith('0')) {
     normalized = '62' + stripped.slice(1);
   } else if (stripped.startsWith('+')) {
     normalized = stripped.slice(1);
+    hasExplicitCountryCode = true;
   } else {
     normalized = stripped;
   }
@@ -25,12 +27,15 @@ export function normalizeWhatsappNumber(raw: string): string | null {
     return normalized;
   }
 
+  if (!hasExplicitCountryCode) return null;
+
   if (normalized.length < 8 || normalized.length > 15) return null;
 
   return normalized;
 }
 
 export function formatWhatsappNumberForInput(normalized: string): string {
+  if (normalized === '') return normalized;
   if (normalized.startsWith('62')) return '0' + normalized.slice(2);
-  return normalized;
+  return '+' + normalized;
 }