My understanding of the unfunded liability portion is this.
Let's take your Comcast example. Suppose the way you purchased cable from Comcast was to contract 30 years in advance, for example every month you promised Comcast that you will purchase $150 of cable from them for one month 30 years from now, and you repeated doing this each month. Now every month that goes by you have a "future liability" of $150, so after one year you have a future liability of $1,800 and after ten years you have a "future liability" of $18,000.
Now for the "unfunded" part, lets say that your income minus other expenses left you with $75 each month. Since you know that you will have to pay Comcast $150 in the future you save that $75 to pay that $150 bill, which is not enough. You now have a "funded liability" of $75 and an "unfunded liability" of $75. This unfunded liability keeps growing and after 30 years you have an unfunded liability of $27,000, but you also have $27,000 saved for the total $54,000 future liability.
This may seem manageable, but it gets very ugly, here's how.
Now 30 years goes by, and Comcast says OK pay me $150 a month now. You have $27,000 saved and $75 coming in each month. This is not enough to pay Comcast, so you do two things: 1) You take the $75 coming in each month and no longer save it but spent it by giving it Comcast and 2) you take $75 from your savings each month and give it to Comcast. Comcast is happy, you promised $150 and they are receiving $150. However, 1) your savings is rapidly dwindling and 2) each month you are still promising $150 a month 30 years from now. Each month your "unfunded liability" keeps growing because you are spending your savings account and no longer saving at all for Comcast's future payments.
After another 30 years goes by you have fully spent your saving account, but Comcast wants $150 each month but you only have $75 coming in. So you start to borrow $75 each month to pay Comcast. Now not only are you no longer saving at all, but you are going into debt. Your unfunded liability here would be $54,000. But everyone is happy, you are getting cable while not saving and Comcast is getting $150 a month. However each month your "unfunded liability" keeps growing, both in terms of the promises to Comcast and promises to the bank.
Many many more years go by. You now owe $1M to the bank, owe $150 to Comcast each month, but only take in $75 each month. This is what the massive unfunded liability number of $125T really represents.
Since you are the government a couple things happen: 1) You stop paying Comcast $150 each month and only give them 50% of what is promised (the $75 you take in). This damages Comcast because they expected $150 and need that. 2) You tell the banks that you will not pay them $1M and default (either through inflation or a refusal to pay, both are the same).
Comcast in this example are Social Security payments to Gen-X, Y and Millennials, Obamacare, Medicare, etc.
Banks in this example are individuals with savings in savings accounts at banks.
Got it?
Ok, so, the good news is that all the unfunded liabilities won't drop onto the balance sheet all at once, the bad news is that its still all eventually debt, it just might be a while and becomes debt at a fairly predictable rate.
Now the question is, what stage are we in. Are we in the (A) still saving $75/mo stage (which apparently is going into not-quite-reducing our national debt) (B) in the spend $75 of our savings stage or (C) in the borrow $75 stage? Also, how much time is there between phases.
Also one other thing I don't get: How are the banks the individuals with savings in savings accounts at banks? Is the theory that they'll lose their money to inflation as the USG borrows more from the Fed to pay the unfunded liabilities?