diff --git a/backend/prisma/migrations/20260727100000_mobile_network/migration.sql b/backend/prisma/migrations/20260727100000_mobile_network/migration.sql
new file mode 100644
index 00000000..abe47cf8
--- /dev/null
+++ b/backend/prisma/migrations/20260727100000_mobile_network/migration.sql
@@ -0,0 +1,6 @@
+-- Mobilfunknetz pro Mobilfunkvertrag (TELEKOM | VODAFONE | TELEFONICA,
+-- NULL = nicht gewählt). String statt Enum, damit weitere Netze ohne
+-- Migration ergänzbar sind.
+
+ALTER TABLE `MobileContractDetails`
+  ADD COLUMN IF NOT EXISTS `mobileNetwork` VARCHAR(191) NULL;
diff --git a/backend/prisma/schema.prisma b/backend/prisma/schema.prisma
index 8b374ab7..4ff4bb23 100644
--- a/backend/prisma/schema.prisma
+++ b/backend/prisma/schema.prisma
@@ -970,6 +970,10 @@ model MobileContractDetails {
   contractId      Int       @unique
   contract        Contract  @relation(fields: [contractId], references: [id], onDelete: Cascade)
   requiresMultisim Boolean  @default(false)  // Multisim erforderlich?
+  // Physisches Mobilfunknetz, auf dem der Tarif läuft: TELEKOM | VODAFONE
+  // | TELEFONICA (null = nicht gewählt). Als String statt Enum gehalten,
+  // damit weitere Netze ohne Migration ergänzbar sind.
+  mobileNetwork   String?
   dataVolume      Float?
   includedMinutes Int?
   includedSMS     Int?
diff --git a/backend/src/services/contract.service.ts b/backend/src/services/contract.service.ts
index 5c7291a9..1ed62d8d 100644
--- a/backend/src/services/contract.service.ts
+++ b/backend/src/services/contract.service.ts
@@ -279,6 +279,7 @@ interface ContractCreateData {
   };
   mobileDetails?: {
     requiresMultisim?: boolean;
+    mobileNetwork?: string | null;
     dataVolume?: number;
     includedMinutes?: number;
     includedSMS?: number;
@@ -386,6 +387,7 @@ export async function createContract(data: ContractCreateData) {
             mobileDetails: {
               create: {
                 requiresMultisim: mobileDetails.requiresMultisim,
+                mobileNetwork: mobileDetails.mobileNetwork ?? null,
                 dataVolume: mobileDetails.dataVolume,
                 includedMinutes: mobileDetails.includedMinutes,
                 includedSMS: mobileDetails.includedSMS,
diff --git a/docs/todo.md b/docs/todo.md
index 3e08e8a8..daf83e59 100644
--- a/docs/todo.md
+++ b/docs/todo.md
@@ -97,6 +97,17 @@ isolierte Instanz (keine Multi-Tenancy im Code), Provisioning + Abrechnung
 
 ## ✅ Erledigt
 
+- [x] **📱 Mobilfunk: Feld „Mobilfunknetz" unter Anbieter & Tarif**
+  - Neues Dropdown „Mobilfunknetz" in der Anbieter-&-Tarif-Karte, nur
+    sichtbar wenn Vertragstyp = Mobilfunk. Optionen: Bitte auswählen
+    (leer), Telekom, Vodafone, Telefónica.
+  - Neues Feld `MobileContractDetails.mobileNetwork` (String, nullable,
+    speichert TELEKOM/VODAFONE/TELEFONICA) + Migration
+    `20260727100000_mobile_network` (`ADD COLUMN IF NOT EXISTS`).
+    String statt Enum, damit weitere Netze ohne Migration möglich sind.
+  - Anzeige in der Vertragsansicht (Mobilfunk-Details) mit lesbarem
+    Netz-Namen.
+
 - [x] **🐞 Lieferbestätigung: eingegebenes Datum ändert Vertragsbeginn nicht**
   - Beim Upload einer Lieferbestätigung mit Datum wurde `startDate` nur
     gesetzt, wenn es noch LEER war (`if (!contract.startDate)`). Hatte
diff --git a/frontend/src/pages/contracts/ContractDetail.tsx b/frontend/src/pages/contracts/ContractDetail.tsx
index c7efcd3b..e212d3cd 100644
--- a/frontend/src/pages/contracts/ContractDetail.tsx
+++ b/frontend/src/pages/contracts/ContractDetail.tsx
@@ -3077,6 +3077,12 @@ export default function ContractDetail() {
       {c.mobileDetails && (
         <Card className="mb-6" title="Mobilfunk-Details">
           <dl className="grid grid-cols-2 md:grid-cols-4 gap-4">
+            {c.mobileDetails.mobileNetwork && (
+              <div>
+                <dt className="text-sm text-gray-500">Mobilfunknetz</dt>
+                <dd>{({ TELEKOM: 'Telekom', VODAFONE: 'Vodafone', TELEFONICA: 'Telefónica' } as Record<string, string>)[c.mobileDetails.mobileNetwork] || c.mobileDetails.mobileNetwork}</dd>
+              </div>
+            )}
             {c.mobileDetails.dataVolume && (
               <div>
                 <dt className="text-sm text-gray-500">Datenvolumen</dt>
diff --git a/frontend/src/pages/contracts/ContractForm.tsx b/frontend/src/pages/contracts/ContractForm.tsx
index 52c5bb32..1ff62374 100644
--- a/frontend/src/pages/contracts/ContractForm.tsx
+++ b/frontend/src/pages/contracts/ContractForm.tsx
@@ -345,6 +345,7 @@ export default function ContractForm() {
         activationCode: c.internetDetails?.activationCode || '',
         // Mobile details
         requiresMultisim: c.mobileDetails?.requiresMultisim || false,
+        mobileNetwork: c.mobileDetails?.mobileNetwork || '',
         dataVolume: c.mobileDetails?.dataVolume || '',
         includedMinutes: c.mobileDetails?.includedMinutes || '',
         includedSMS: c.mobileDetails?.includedSMS || '',
@@ -656,6 +657,7 @@ export default function ContractForm() {
     if (data.type === 'MOBILE') {
       contractData.mobileDetails = {
         requiresMultisim: data.requiresMultisim || false,
+        mobileNetwork: emptyToNull(data.mobileNetwork),
         dataVolume: data.dataVolume ? parseFloat(data.dataVolume) : null,
         includedMinutes: safeParseInt(data.includedMinutes) ?? null,
         includedSMS: safeParseInt(data.includedSMS) ?? null,
@@ -1019,6 +1021,18 @@ export default function ContractForm() {
               options={availableTariffs.map((t) => ({ value: t.id, label: t.name }))}
               disabled={!selectedProviderId}
             />
+            {contractType === 'MOBILE' && (
+              <Select
+                label="Mobilfunknetz"
+                {...register('mobileNetwork')}
+                options={[
+                  { value: 'TELEKOM', label: 'Telekom' },
+                  { value: 'VODAFONE', label: 'Vodafone' },
+                  { value: 'TELEFONICA', label: 'Telefónica' },
+                ]}
+                placeholder="Bitte auswählen"
+              />
+            )}
             <Input label="Kundennummer beim Anbieter" maxLength={100} {...register('customerNumberAtProvider')} />
             <Input label="Vertragsnummer beim Anbieter" maxLength={100} {...register('contractNumberAtProvider')} />
             <Input label="Auftragsnummer bei Vertriebsplattform" maxLength={100} {...register('orderNumberAtSalesPlatform')} />
diff --git a/frontend/src/types/index.ts b/frontend/src/types/index.ts
index f3a62060..84e2c049 100644
--- a/frontend/src/types/index.ts
+++ b/frontend/src/types/index.ts
@@ -588,6 +588,8 @@ export interface MobileContractDetails {
   id: number;
   contractId: number;
   requiresMultisim?: boolean;
+  /** Physisches Mobilfunknetz: TELEKOM | VODAFONE | TELEFONICA (oder null). */
+  mobileNetwork?: string | null;
   dataVolume?: number;
   includedMinutes?: number;
   includedSMS?: number;