A method for detection of a bypass (134; 734; 834) of an interconnect (133; 733; 833) to a telecommunication network (130; 730; 830), comprising: - generating a test call (263; 367; 467; 567; 663) to or through a real subscriber number of a real subscriber (131; 231; 331; 431; 531; 631; 731; 831) active in said telecommunication network; - obtaining call data (153; 265; 372; 472; 572; 665; 753; 853) from a test probe (132; 232; 332; 432; 532; 632; 732; 832) whereto said real subscriber number of said real subscriber (131; 231; 331; 431; 531; 631; 731; 831) is temporarily forwarded during a time interval wherein said test call (263; 367; 467; 567; 663) is made, said call data (153; 265; 372; 472; 572; 665; 753; 853) being reported by said test probe (132; 232; 332; 432; 532; 632; 732; 832) and including a received calling line identifier corresponding to the calling line identifier that would have been received by said real subscriber (131; 231; 331; 431; 531; 631; 731; 831) if its number was not configured in call forward mode; - extracting a received calling line identifier for said test call (263; 367; 467; 567; 663) from said call data (154, 153; 265; 372; 472; 572; 665; 753; 853) comprising extracting said received calling line identifier from said call data (153; 265; 372; 472; 572; 665; 753; 853) reported by said test probe (132; 232; 332; 432; 532; 632; 732; 832); and - detecting presence of a bypass (134; 734; 834) by analysis of said received calling line identifier, by verifying if said received calling line identifier belongs to the numbering plan of said telecommunication network (130; 730; 830) wherein said test call (263; 367; 467; 567; 663) is received, or by verifying the country code, or by establishing that said received calling line identifier does not correspond to the calling line identifier of an origin of said test call (263; 367; 467; 567; 663). - generating a test call (263; 367; 467; 567; 663) to or through a real subscriber number of a real subscriber (131; 231; 331; 431; 531; 631; 731; 831) active in said telecommunication network; - obtaining call data (153; 265; 372; 472; 572; 665; 753; 853) from a test probe (132; 232; 332; 432; 532; 632; 732; 832) whereto said real subscriber number of said real subscriber (131; 231; 331; 431; 531; 631; 731; 831) is temporarily forwarded during a time interval wherein said test call (263; 367; 467; 567; 663) is made, said call data (153; 265; 372; 472; 572; 665; 753; 853) being reported by said test probe (132; 232; 332; 432; 532; 632; 732; 832) and including a received calling line identifier corresponding to the calling line identifier that would have been received by said real subscriber (131; 231; 331; 431; 531; 631; 731; 831) if its number was not configured in call forward mode; - extracting a received calling line identifier for said test call (263; 367; 467; 567; 663) from said call data (154, 153; 265; 372; 472; 572; 665; 753; 853) comprising extracting said received calling line identifier from said call data (153; 265; 372; 472; 572; 665; 753; 853) reported by said test probe (132; 232; 332; 432; 532; 632; 732; 832); and - detecting presence of a bypass (134; 734; 834) by analysis of said received calling line identifier, by verifying if said received calling line identifier belongs to the numbering plan of said telecommunication network (130; 730; 830) wherein said test call (263; 367; 467; 567; 663) is received, or by verifying the country code, or by establishing that said received calling line identifier does not correspond to the calling line identifier of an origin of said test call (263; 367; 467; 567; 663).