Thanks for raising this with us.
We've tested the line and was not able to detect any fault within the external wiring, but there's a possible filtering issue.
If both modem and handset is connected to a splitter/filter, then try to use another splitter/filter.
You may need to try another handset or phone cable to rule out a possible device issue.
If you have multiple wall sockets, then connect both modem and handset separately without a splitter/filter, then try to perform a test call to check if the internet will drop out.
Let us know how it goes.