Yeap
you can use your grandma's DOB
I did just that.
Amazingly, this service works perfectly*.
Many thanks to the author of the thread!
I just hope my bill stays the same.
For the record, I called a Bulgarian Landline from my O2 number (which should be free). The duration of the phone call was 8'10''. My mum (who was at my grandma's at the time) picked up pretty quickly (there was no dialling delay I mean) and the connection was crystal clean (more or less).
Will update 'my bill status' info tomorrow, as 12 hours is the maximum time for my (pay monthly) bill to refresh.
Follow-up:
18 Sep 10 18:36:57 075******** Weekend 00:08:12 0.000 F
Hooray!
*as strange as it may seem, I can call my grandma but not my mum, although they both have landlines, as well as mobiles (however, calling mobiles in Bulgaria is not worth it - £1/min is money down the pan; better call direct - 17 ppm).