Lately, many people complain that they are call from unknown numbers, from remote countries. If it is to get us after prefix and after the country is identified by the mobile phone, these phone numbers belong to countries in Central Africa, South Africa or exotic countries removed.
Later we have received such calls from the following numbers: + 9606878121 (Maldives), + 23590049941 (Chad), + 265998889952 (Malawi)
If we answer these calls on unknown phone numbers, Most often a music is heard or voice in an unknown language Europeans. In most cases, the phone rings only once or twice, after which the call closes before we answer.
It may also be the purpose of the calls. Give some kind of "beeps", then call him back. There are great chances these phone numbers are surcharges.
So even if you are subscribed to Orange, Vodafone or Telekom, it is best do not answer to such numbers if you know there is not really anyone to contact you from remote countries and especially, do not call back to international phone numbers you do not know. You risk having a substantial additional cost at bill for mobile phone service.