Dial tone is 2 tone - what does it mean?

[crossedwires]

My dial tone is now 2 tone. It is a bit like a old fashioned police car perhaps?

What does it mean?

What do I have to do to get rid of it (if the problem could be different causes, what are the different things I have to do)?

Also, is this documented online anywhere so I can understand for myself. I wasn't expecting it.

Many thanks for your help.

[rbz5416 · Accepted solution]

It means the BT Voicemail service has a message recorded. Dial 1571 to hear/clear it & the dial tine will return to  normal after.