Landline: static and immediate disconnection

My workplace has a really basic landline. Last week, we had an issue where anyone who called would – when the phone was answered – get an earful of static for a second, and then be disconnected. Outgoing calls could be placed without issue. The issue went away overnight and we chalked it up to a temporary glitch. Now, about a week later, we're having the same problem.

Any thoughts/ideas/suggestions?