docs: add didlogic SIP trunk integration guide

fern/advanced/sip/sip-didlogic.mdx

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+---
+title: "didlogic SIP Integration"
+description: "How to integrate your didlogic SIP trunks and phone numbers with Vapi AI Voice Agents"
+slug: advanced/sip/didlogic
+---
+
+## Overview
+
+For a general introduction to SIP trunking with Vapi (concepts and architecture), see our [SIP Trunking Guide](../sip-trunk.mdx).
+
+To connect didlogic with Vapi, you will set up a two-way SIP connection:
+1. **Outbound Calls (Vapi → didlogic):** Vapi sends call requests to didlogic via SIP Trunking.
+2. **Inbound Calls (didlogic → Vapi):** didlogic routes incoming calls on your purchased numbers to Vapi's SIP URI.
+
+## Prerequisites
+
+Before getting started, make sure you have:
+* An active [didlogic account](https://didlogic.com/get-started?utm_source=vapi_docs) with a positive balance.
+* At least one purchased phone number in your didlogic dashboard.
+* An active SIP account in your didlogic dashboard.
+* A **Vapi** account with access to the Dashboard.
+
+## Get Started
+
+### Step 1: Configure Outbound Trunking & Import didlogic Numbers to Vapi
+
+To allow your Vapi agents to place outbound calls through didlogic:
+
+1. Log in to your **Vapi Dashboard** and navigate to **Integrations**.
+2. Find the **Phone Number Providers** section and select **SIP Trunk → Configure New SIP Trunk**.
+3. Fill in your didlogic SIP Trunk credentials:
+   * **Outbound SIP Gateway:** `sip.nl.didlogic.net` ([full list of regional gateways](https://docs.didlogic.com/docs/guides/getting-started/outbound-calling#our-sip-gateways))
+   * **Username:** Your 5-digit didlogic SIP Username
+   * **Password:** Your didlogic SIP Password
+4. Save the configuration.
+5. Go to [Phone Numbers](https://dashboard.vapi.ai/phone-numbers) → Create Phone Number → BYO SIP Trunk Number.
+6. Enter your didlogic number and select your didlogic SIP trunk in the **SIP Trunk Credential** dropdown. Then, save changes.
+
+### Step 2: Configure Inbound Call Routing in didlogic
+
+To route incoming calls on your didlogic numbers to a Vapi agent:
+
+1. Go to [Vapi API keys](https://dashboard.vapi.ai/org/api-keys) and copy your **Private Key**.
+2. Log in to the [didlogic Customer Portal](https://app.didlogic.com).
+3. Navigate to **Purchased**.
+4. Find the phone number you want to assign to Vapi and click **Edit** in the Destination block.
+4. Set the **Destination Type** to `SIP URI`.
+5. Enter your Vapi Inbound SIP URI target:
+   ```code
+	YOUR_PHONE_NUMBER@VAPI_PRIVATE_KEY.sip.vapi.ai // e.g. 31203691111@123a-456b-789c.sip.vapi.ai
+6. Click Add.
+
+### Step 3: Test the Integration
+
+**Outbound Test**
+1. In Vapi, select an agent assigned to your newly created didlogic SIP Trunk.
+2. Trigger a test call to your personal phone number.
+3. Verify that the call connects and the caller ID displays correctly.
+
+**Inbound Test**
+1. Dial your didlogic phone number from an external phone.
+2. Ensure Vapi receives the call and your configured AI agent answers immediately.
+
+### Step 4: Configure SIP REFER (Optional)
+
+To enable the "Transfer to Human" feature, SIP REFER should be set up in Vapi.
+
+<Callout type="warning">
+* SIP REFER must be activated for your didlogic account. Contact your account manager to enable SIP REFER before proceeding with the configuration.
+* Transferring is technically possible to your other didlogic numbers only. Set up the correct routing for the number that you use for referring.
+</Callout>
+
+1. Create the **Transfer Call** tool.
+2. Navigate to Destinations → Add destination → Phone Number.
+3. Enter the target phone number and an optional message for the customer.
+4. Save changes.