Received one call on Sunday at 5:15 a.m. and another today at 6:30. I answered today and there was no one on the line. I called back today and got the same "high volume of calls" and music.
For 2 days I received phone-calls at 05:30 in the morning. Calling back it says all lines are busy "high volume of calls" and start to play music. Maybe they try to figure out if somebody is at home to prepare a burglary or try to make money using expensive lines.