Phone verification can fail for several reasons. Here are some common causes:

### 1. **Incorrect Phone Number**  
   - Typos or wrong country codes can prevent the verification code from being sent.  
   - Using an unsupported or virtual number (e.g., VoIP services like Google Voice, which some platforms block).  

### 2. **Network or Carrier Issues**  
   - Poor signal or mobile network problems can delay or block SMS delivery.  
   - Carrier restrictions (some networks filter automated messages).  

### 3. **App or Service Limitations**  
   - The verification service may have temporary outages or high traffic.  
   - The platform might block repeated attempts (rate limiting).  

### 4. **SMS Filtering & Spam Blocking**  
   - Some phones automatically filter verification texts into spam folders.  
   - Carrier-level spam blocking may interfere with delivery.  

### 5. **Timeouts & Expired Codes**  
   - Verification codes often expire after a few minutes; delays in SMS delivery can render them invalid.