Thieves stole a mile of BT phone cable after lifting eight manhole covers between Thelveton and Scole on Wednesday.
The incident happened in Norwich Road overnight and police believe the thieves would have spent a number of hours carrying out their crime.
Police said the offenders would have needed a lot of equipment and could have been mistaken for legitimate workers.
Anyone who may have witnessed people working in the area, tampering with manhole covers or suspicious vehicles should phone the Diss Safer Neighbourhood Team on 101 or Crimestoppers on 0800 555111.
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