C2C issue: Object reference not set to an instance of an object

Checklist :

1. The URL being used should not have mistakes or extra spaces. Ensure it is exactly as shared by the provider.

2. Try changing the request-response type to Text.

3. The response type/body in C2C configuration should not be a nested JSON. It should typically be in the following format:
   {"status": "success", "message": "call initiated"}

4. Confirm if the telephony provider has whitelisted all the following IPs:

3.7.140.178  
13.126.22.232  
13.127.46.179  
13.200.164.32 (for ULC/UDS)  
13.232.49.129  
13.234.155.250  
13.250.244.76  
13.58.85.228  
13.58.241.213  
15.206.113.19  
15.207.93.208 (for ULC/UDS)  
34.248.218.80  
43.204.86.147 (for ULC Legacy System)  
52.16.26.19  
52.66.49.250  
52.66.85.71  
52.76.2.151  
52.76.12.168  
54.247.175.231  
54.75.227.209