Sprint Corp. (NYSE: S) is throwing the book at four cable operators, including three long-standing partners, for allegedly infringing a dozen patents related to VoIP services.

On Monday, Sprint filed separate lawsuits against Time Warner Cable Inc. (NYSE: TWC), Comcast Corp. (Nasdaq: CMCSA, CMCSK), Cox Communications Inc. and CableOne in the U.S. District Court for the District of Kansas, Bloomberg reported. Sprint included a number of patents that it asserted against Vonage Holdings Corp. (NYSE: VG) in 2007; Vonage ended up paying $80 million to license the technology from Sprint. (See Vonage Soars on Sprint Settlement.)

TW Cable and Cox officials declined to comment. Comcast and CableOne could not immediately be reached Monday night.

Why this matters
The timing of the filings is curious, considering TW Cable, Comcast and, most recently, Cox, just agreed to sell wireless spectrum to Verizon Wireless and strike a multi-faceted services and technology partnership with the mobile carrier. TW Cable and Comcast are also phasing out sales of Sprint's 3G data services and Clearwire LLC (Nasdaq: CLWR)'s WiMax. (See VZ Wireless Nabs Cox's AWS Spectrum for $315M and VZ Wireless Nabs Cox's AWS Spectrum for $315M .)

Some of Sprint's ire might also be directed at TW Cable, which decided in 2010 to stop using Sprint for VoIP transport and interconnections to the tune of $10 or $11 per subscriber per month. TW Cable is taking those components in-house to save money. (See Time Warner Cable to 'Go It Alone' With VoIP .)

Sprint is also targeting these MSOs now that VoIP has become a big business for cable. Comcast had 9.2 million VoIP subscribers, and TW Cable had 4.5 million such subs at the end of the third quarter, representing $883 million and $494 million in revenues, respectively, during that period. (CableOne and Cox are privately held and don't give out these numbers so freely.)

For more
Read more about Sprint's up and down relationship with cable.

— Jeff Baumgartner, Site Editor, Light Reading Cable