Landline doesn't work sometimes, without a pattern

This is a problem with my landline which comes and goes without a pattern. My line has no dial tone and I cannot make any calls. I cannot receive calls either as it appears busy. Call forwarding works when set to 0 rings. The online tool can find no problem with the network in the area or the settings. I have tried the following:

Restart modem or telephone

Change cable

When disconnecting my telephone device completely line still apperas busy when I call

The problem persists for hours and maybe suddenly resolved by itself and come back later or another day.

Thank you in advance for your help