The letter has also gone to Bypass Network Services - the small Auckland firm that created the global mode technology used by Slingshot and Orcon.
Broadcasters believe Global Mode breaches their local copyright on TV shows and movies by allowing customers to set up false addresses.
It provides "back door" and easy access to US pay-TV services.
Horth said M2 does not provide Global Mode across the Tasman, which allows phone companies to provide easy access.
"That conduct under Australian laws would be seen to be aiding and abetting. " Horth said though some companies were offering proxy accessing overseas networks by using Virtual Private Networks.," he said,
VPNs are less consumer-friendly.
Horth declined to discuss whether the negotiated sale price incorporated the revenue from Global Mode or, more recently, contingent liabilities from a potential legal challenge as a result of the warning letter.
Call Plus co-founder chief executive Mark Callander could not be reached for comment.
Yesterday he said: "It's a shared purchase so obviously M2 will be the ultimate owner of the business, so for now its business as usual but obviously once everything is completed it will be an M2 matter for consideration."
"Now this is out of the way it's business as usual, we'll go back to, and have a look at, the letters we've received and we'll respond in due course but it really is business as usual from our perspective."
John Moore of Miro Capital Advisory said Call Plus was an agreed sale and he was not privy to the terms.
"They may have a plan to drop it anyway and that may be a factor in the price they are paying - but the reality is they are not going to do that until they are in control.
Sky TV chief executive John Fellet said that Global Mode had "hammered away" at the market.
He said that other companies - like Vodafone - were using content to drive their sales.
"Call Plus without wanting to do one of those deals decided to go their own way," he said.
He said that the heavy promotion was a factor in the dispute but the fundamental issue was that the four broadcasters involved all relied on copyright for their business.
Advocates for open source access to the internet have slammed the broadcasters' approach, saying that they are undermining consumer freedom.
