I have tested your NBN-FTTP 50/20 service on our portals and all tests are passing. I can see your phone registered.
Best thing to do is test your handset DIRECT to the UNI-V1 port, and bypass any long line cord lengths.
If it still cuts out, you may need to use another phone for trouble-shooting. Some certain types of phones can be problematic on certain NBN product types.