Telstra has warned the federal government it has massively overstated the value of multibillion-dollar spectrum licences, issuing a thinly veiled threat that it would be prepared to mount a legal challenge to avoid overpaying.The telecommunications sector is on tenterhooks this week because the regulator, the Australian Communications and Media Authority, is expected to issue a final ruling on how much to charge Telstra, Optus, TPG Telecom and NBN Co to renew licences to use the radio waves that transmit mobile phone calls, known as spectrum.