Hi, friend.
HG520C is a carrier product.
You can try the following methods to troubleshoot the fault:
Step 1: Ensure that the terminal and other devices, such as telephones or fax machines, are connected to telephone lines through a splitter.
Step 2: Ensure that telephone lines are properly connected. If the telephone lines are improperly connected, the stability of the network connection is affected.
Step 3: Check the positions of your terminal and your computer. Ensure that they are far from electric appliances, such as microwave ovens, refrigerators, or cordless telephones, that generate strong magnetic or electric fields.
If the problem persists, contact your service provider.
Thanks.