Cleveland Police used anti-terror powers as part of an investigation into alleged leaking of confidential material to newspapers in 2012.
However, the force claimed it was justified in secretly monitoring the phones of two former officers.
Mark Dias and Steve Matthews claim they were targeted after challenging misconduct by fellow officers.
The decision over whether the force's covert techniques used against its former employees were lawful will be made in the new year, following an Investigatory Powers Tribunal.
At the hearing in London, Judge Sir Michael Burton said although he reserved judgement until 2017, it was clear what his decision would be.
He said the force should start thinking about compensation for the two officers.
Cleveland Police claimed it was justified in using the Regulation of Investigatory Powers Act (RIPA) - which has since been replaced.
In a four-month period the force monitored over a million minutes of telephone data, the tribunal heard.
Mr Matthews, the former head of Cleveland's Police Federation, said: "I honestly think they were trying to stitch me up."
If anyone criticised the force "they would be jumped on and, as part of that campaign, I was singled out", he said.
Mr Dias said the force monitored his phone calls, emails and activity.
The officers' legal team said the "force's casual disregard for the legal framework beggared belief".

Dyfed-Powys Police said its officers made arrests relating to controlled drugs at the dance party near Llanddewi Brefi over the bank holiday weekend.
Local people and councillors have called for action to ensure more raves are not held.
Organisers of the event had relayed the remote location to people via text and email rather than social media.
Police said they became aware of a "large gathering" late on Saturday, 28 May but because it was already on a  "a very large scale" the decision was made not to break up the party.
Instead, officers stayed at the event to ensure more people could not enter the site.
The police helicopter helped to gather intelligence and evidence and arrests were made.
Llanddewi Brefi councillor Rhodri Evans said: "I have had quite a few people in Llandewi expressing their concerns - people driving through the village at very high speeds and perhaps putting people in danger."
He added that the area was "quiet and peaceful" and villagers wanted to ensure more raves were not held.
Officers have urged landowners to be vigilant to people enquiring about their land in the guise of hiring it for apparently acceptable activities such as gymkhanas and scouts and guides events.

Pro Rugby America, in a statement, described Pro12 as an "interloper" and claimed the league "does not have a legitimate business plan."
Pro12 officials held talks about US and Canadian sides being involved, which had been backed by World Rugby.
"It's ridiculous. What they're doing is just disrespectful," Pro Rugby America chief Doug Shoninger told BBC Wales.
"I understand Pro 12 is under a lot of stress, a lot of problem, but it's not our problem. And they need to solve that problem where they sit."
Pro12 is made up of four teams each from Wales and Ireland plus two each from Scotland and Italy.
Pro Rugby America is sanctioned by governing body USA Rugby to run a professional rugby division in the United States.
The US league started in April with five teams based in Denver, Ohio, Sacramento, San Diego and San Francisco and Shoninger told BBC Radio Wales they were negotiating a deal with the southern hemisphere Super Rugby league.
"We're bidding on a Super Rugby team," he said.
"We're talking to big established countries, ones I feel that have player exchange at academy levels.
"We're very honest about who we are - we have the potential great commercialism and we need the support of the rugby world to build up our infrastructure."
Pro12 is commercially smaller than the English and French professional leagues, and in August, Scottish Rugby chief Mark Dodson said they were "seriously investigating" the possibility of expanding into the American market.
The two Italian teams that compete in the league, Zebre and Treviso, have faced calls to be dropped from the Pro12 and were almost expelled in 2015 over money owed to the rest of the teams.
Pro12 Rugby declined to comment.
A police force has admitted to unlawfully accessing the private phone records of journalists and a solicitor.
[NEXT_CONCEPT]
Police are investigating after about 2,000 people turned up for a three-day illegal rave in Ceredigion.
[NEXT_CONCEPT]
Plans for American sides to join the Pro12 have been dismissed by the company running the pro game in the US.