Verizon boss Lowell McAdam, his company facing slowing sales of mobile phones, made a proposal to acquire cable TV giant Charter Communications in recent months, three sources told The Post.

The offer — valued at between $350 and $400 a share, and well over $100 billion, according to two of the sources familiar with the move — was rejected by Charter because it was too low — and because Charter and its largest shareholder, Liberty Media, weren’t ready to sell.

Verizon, whose archrival AT&T has moved to expand beyond the wireless world by buying DirecTV and Time Warner, also recently expressed interest in another Liberty Media property, Sirius XM Holdings, sources said.

Verizon’s interest in SiriusXM didn’t get as far as a bid, the sources said.

Also standing in the way of Liberty Media agreeing to a deal for any of its units is the tax implications, which would be unpalatable to its billionaire chairman John Malone, sources said.

Malone is famous for avoiding taxes in big transactions.

In addition, Malone wants to give Stamford, Conn.-based Charter some time to complete the integration of its acquisition of Time Warner Cable, sources said.

Charter’s stock has been on a tear since Liberty took a stake in the firm in March 2013 and drafted former Cablevision chief operating officer Tom Rutledge as the boss. The shares have soared roughly 262 percent through Wednesday’s close at $345.55, up 2 percent on the day.

Verizon, whose interest in acquiring a cable giant like Charter or Comcast was reported by The Post in January, is scrambling to grow its scale in a rapidly consolidating sector.

As growth at the wireless giant’s phone business stalls, it has been looking to advance its online advertising business by focusing on phone-obsessed millennials.

Verizon will close on its acquisition of Yahoo next month and is planning to merge its operations with AOL under Tim Armstrong. Still, CEO McAdam needs something more transformative now that he’s facing off against rival AT&T and its proposed $85 billion get-together with Time Warner.

Verizon is eyeing a big cable acquisition to help it launch its high-speed 5G wireless plans, which require the kind of fiber-optic networks operated by cable.

Meanwhile, an agreement Verizon cut years ago to lease wireless spectrum to the cable industry for mobile phone services may soon come back to haunt it. Earlier this month, Charter and Comcast signed a deal to work together to launch their own wireless phone plans.

Some sources also speculate that the odd coupling of archrivals Charter and Comcast is meant to prod Verizon into a high-priced cable acquisition that would boost cable stocks as well as create a viable competitor to AT&T and Time Warner.

Back in December, McAdam said a get-together with Charter made “industrial sense.”

Verizon’s name crops up frequently in conjunction with properties for sale and some suggest it also would be a good fit with online radio giant Pandora.

Verizon shares added 1 percent Wednesday to close at $46.64. Verizon declined comment.

Additional reporting by Richard Morgan