Orange internet telephone doing strange things - anyone suffered similar?

[Daft Doctor]

Hi, I've asked Orange for an answer to this problem but they gave me the telephone equivalent of a gallic shrug.  We have internet telephone from Orange without a France telecom line rental.  Mainly all is well and the free calls to UK landlines are great, with excellent clarity both ends.  Got my Panasonic DECT wireless system working easily (question posed in earlier thread).  Thing is that an average of roughly once a day (anytime between 0700 and 2230) the phone rings as normal, but on pick up all there is to hear is a series of beeps, rather like a slowed down version of the UK engaged tone.  The caller id simply displays 'private number' but there is never anyone on the other end of the phone when it happens.  Has anyone else had similar 'symptoms' and does anyone have the foggiest why it is happening and, crucially, whether there is anything that can be done about it from our end?  Thanks in advance, any help as always greatly appreciated   

[pachapapa]

A trick of telephone marketing; they ring to see if youn are there; later they ring again with an experienced marketeer.

The number may show up in the router admistration page in section calls received since last reboot.

[pachapapa]

As an example of the use of the router, I have this morning received a call replying to a le bon coin email query in respect of the purchase of a pair of shock absorbers. The calls journal records the following:

10/05/2012 10:11 0430150220 THOMAS JEAN PAUL 24 minutes

After 24 minutes all the details of the suitability of the piece and delivery together with the useful knowledge that he has a 300 M2 store full of spare parts for older vehicles.

But the main point is that pertinent details of the call are noted in addition to "rogue" calls mentioned earlier.