The hackers may well have been either the NSA or the UK's GCHQ.
"For Belgacom, the protection of the customers and their data is a key priority," the company said in a statement. "At this stage there is no indication of any impact on the customers or their data. At no point in time has the delivery of our telecommunication services been compromised."
All employees are being required to change their passwords, and the company says it has "filed a complaint against an unknown third party," and is working with Belgium's Federal Prosecutor to investigate the incident.
GigaOm's David Meyer reports that the attacker was most likely either the NSA or the UK's GCHQ -- the hackers appear to have accessed at least two years of international voice traffic, and were specifically targeting traffic from countries such as Syria and Yemen.