diff --git a/server/user.go b/server/user.go
index 943541e..b0e107a 100644
--- a/server/user.go
+++ b/server/user.go
@@ -156,6 +156,14 @@ func replyCreateUser(s *Session, msg *ClientComMessage, rec *auth.Rec) {
 				continue
 			}
 			challengeID, _ := cr.Params["challenge_id"].(string)
+			if strings.TrimSpace(challengeID) == "" && phoneRegistration && isLegacyRegistrationSMSCode(cr.Response) {
+				if err := attachLegacyRegistrationSMSChallenge(s.tenantID, cr, phone, time.Now().UTC().Round(time.Millisecond)); err != nil {
+					s.queueOut(decodeStoreError(types.ErrCredentials, msg.Id, msg.Timestamp,
+						map[string]any{"creds": []string{"tel"}}))
+					return
+				}
+				challengeID, _ = cr.Params["challenge_id"].(string)
+			}
 			if phone == "" || strings.TrimSpace(challengeID) == "" || strings.TrimSpace(cr.Response) == "" ||
 				(registrationPhone != "" && registrationPhone != phone) {
 				s.queueOut(decodeStoreError(types.ErrCredentials, msg.Id, msg.Timestamp,
@@ -400,7 +408,15 @@ var (
 
 func validateAccountRegistrationTenant(s *Session, msg *ClientComMessage) error {
 	if msg == nil || msg.Acc == nil || msg.Acc.RegistrationType == nil {
-		return errRegistrationTypeRequired
+		if msg != nil && msg.Acc != nil && s != nil && !strings.EqualFold(s.tenantCode, imDefaultTenantCode) {
+			registrationType := registrationTypeEnterprise
+			msg.Acc.RegistrationType = &registrationType
+			if strings.TrimSpace(msg.Acc.TenantCode) == "" {
+				msg.Acc.TenantCode = s.tenantCode
+			}
+		} else {
+			return errRegistrationTypeRequired
+		}
 	}
 	tenantCode := strings.TrimSpace(msg.Acc.TenantCode)
 	switch *msg.Acc.RegistrationType {
@@ -427,6 +443,51 @@ func validateAccountRegistrationTenant(s *Session, msg *ClientComMessage) error
 	return nil
 }
 
+func isLegacyRegistrationSMSCode(value string) bool {
+	if len(value) != imVerificationCodeLength {
+		return false
+	}
+	for i := range value {
+		if value[i] < '0' || value[i] > '9' {
+			return false
+		}
+	}
+	return true
+}
+
+func attachLegacyRegistrationSMSChallenge(tenantID types.TenantID, cr *MsgCredClient, phone string, now time.Time) error {
+	if store.Verifications == nil {
+		return types.ErrInternal
+	}
+	phoneHash := hashIMSecret(tenantID, "register", phone, "phone")
+	vc, err := store.Verifications.GetActive(tenantID, "register", phoneHash, now)
+	if err != nil {
+		return err
+	}
+	if vc.CodeHash != hashIMSecret(tenantID, vc.Scene, vc.ChallengeID, vc.PhoneHash, strings.TrimSpace(cr.Response)) {
+		nextState := types.VerificationStatePending
+		if vc.Attempts+1 >= vc.MaxAttempts {
+			nextState = types.VerificationStateLocked
+		}
+		_ = store.Verifications.Fail(tenantID, vc.ChallengeID, nextState)
+		return types.ErrInvalidResponse
+	}
+	token, err := randomIMToken(32)
+	if err != nil {
+		return err
+	}
+	tokenHash := hashIMSecret(tenantID, vc.Scene, vc.ChallengeID, vc.PhoneHash, token)
+	if err = store.Verifications.MarkVerified(tenantID, vc.ChallengeID, tokenHash, now); err != nil {
+		return err
+	}
+	if cr.Params == nil {
+		cr.Params = map[string]any{}
+	}
+	cr.Params["challenge_id"] = vc.ChallengeID
+	cr.Response = token
+	return nil
+}
+
 func registrationTenantErrorParams(err error) map[string]any {
 	params := map[string]any{"what": "registration_type", "reason": "REGISTRATION_TYPE_INVALID"}
 	switch {
diff --git a/server/user_registration_test.go b/server/user_registration_test.go
index daab4b3..29a17f2 100644
--- a/server/user_registration_test.go
+++ b/server/user_registration_test.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"regexp"
+	"strings"
 	"testing"
 	"time"
 
@@ -83,6 +84,70 @@ func TestRegistrationSecretForAccount(t *testing.T) {
 	}
 }
 
+func TestValidateAccountRegistrationTenantDefaultsEnterpriseFromSession(t *testing.T) {
+	msg := &ClientComMessage{Acc: &MsgClientAcc{}}
+	s := &Session{tenantCode: "system"}
+
+	if err := validateAccountRegistrationTenant(s, msg); err != nil {
+		t.Fatalf("validateAccountRegistrationTenant returned %v", err)
+	}
+	if msg.Acc.RegistrationType == nil || *msg.Acc.RegistrationType != registrationTypeEnterprise {
+		t.Fatalf("registration type was not defaulted to enterprise: %#v", msg.Acc.RegistrationType)
+	}
+	if msg.Acc.TenantCode != "system" {
+		t.Fatalf("tenant code default = %q, want system", msg.Acc.TenantCode)
+	}
+}
+
+func TestValidateAccountRegistrationTenantStillRequiresPersonalType(t *testing.T) {
+	err := validateAccountRegistrationTenant(&Session{tenantCode: imDefaultTenantCode}, &ClientComMessage{Acc: &MsgClientAcc{}})
+	if err != errRegistrationTypeRequired {
+		t.Fatalf("got %v, want registration type required", err)
+	}
+}
+
+func TestAttachLegacyRegistrationSMSChallenge(t *testing.T) {
+	oldVerifications := store.Verifications
+	defer func() {
+		store.Verifications = oldVerifications
+	}()
+
+	tenantID := types.TenantID(7)
+	phone := "+8618612345678"
+	code := "123456"
+	phoneHash := hashIMSecret(tenantID, "register", phone, "phone")
+	challengeID := "challenge-legacy"
+	store.Verifications = &testVerificationStore{active: &types.VerificationCode{
+		TenantID:    tenantID,
+		ChallengeID: challengeID,
+		Scene:       "register",
+		PhoneHash:   phoneHash,
+		CodeHash:    hashIMSecret(tenantID, "register", challengeID, phoneHash, code),
+		State:       types.VerificationStatePending,
+		MaxAttempts: 5,
+		ExpiresAt:   time.Now().UTC().Add(time.Minute),
+	}}
+
+	cred := &MsgCredClient{Method: "tel", Value: phone, Response: code}
+	if err := attachLegacyRegistrationSMSChallenge(tenantID, cred, phone, time.Now().UTC()); err != nil {
+		t.Fatalf("attachLegacyRegistrationSMSChallenge returned %v", err)
+	}
+	if cred.Params["challenge_id"] != challengeID {
+		t.Fatalf("challenge id = %#v, want %q", cred.Params["challenge_id"], challengeID)
+	}
+	if cred.Response == code || strings.TrimSpace(cred.Response) == "" {
+		t.Fatalf("legacy code was not replaced by verification token: %q", cred.Response)
+	}
+	verifications := store.Verifications.(*testVerificationStore)
+	if !verifications.verified {
+		t.Fatal("verification challenge was not marked verified")
+	}
+	wantHash := hashIMSecret(tenantID, "register", challengeID, phoneHash, cred.Response)
+	if verifications.tokenHash != wantHash {
+		t.Fatalf("token hash mismatch")
+	}
+}
+
 func TestRegistrationPublicWithUsername(t *testing.T) {
 	if got := registrationPublicWithUsername(nil, "alice").(map[string]any)["fn"]; got != "alice" {
 		t.Fatalf("got fn=%v", got)