I like to know if two lines came int a customer premise and both were accidentally terminated together..what affect would it have on the circuit at the central office?

Blow the coil on both circuits.

As a retired central office tech, I will tell you they would both go out of service after about 20 seconds depending how they are crossed will determine how they go out, may be for battery, cross or ground or MTNCE. They will stay down until the problem is removed. This is a common occurrence, not as you state, but wet cables, dug up shorted cables etc cause the same issues for the oos. As a tech, the list can be pulled. Checking addresses on the lines out can easily tell the tech that a problem is external of the office and possibly a wet or damaged cable.

Absolutely no effect at all. Heat coils are only for lightning protection in the central office. Since both lines originate from the same power source in the central office, there will be no harm done. Granted, if the polarity isn't correct on the lines, they will both be busied out because the grounded positive (tip) side of one line is connected to the negative (ring) side of the other line.

If the polarity of both lines is correct, then the only harm done is that when you go to make a call, you'll be calling the same number twice. One of them will get through. The other one will get a busy signal or voice mail.

A call INTO a pair of lines from the same CO that are shorted together will likely result in ring trip, but that's all.

If a fake line (Cable, VoIP, etc.) and a CO line are crossed, they again will just place shorts on each other, marking themselves busy.

Keep in mind that many 1A1, 1A2 and Comkey systems provided conference calling by simply connecting the tip/ring of two lines together under the control of the conference originator. Usually, this was done by replacing the screws in the key strip on the phone with 'conference' screws. They allowed two or more line keys to be depressed simultaneously. Using these screws, even intercom paths could be conferenced with CO lines.

YES, these arrangements caused many trouble reports when people would hang up and not restore the buttons to their unused positions.

With the more modern 1A-type systems, like Comkey, these buttons would pop back up and disconnect the lines once the handset was restored. With older systems, users had to remember to press the hold button (or any other line key) when a conference call was completed in order to restore the line buttons to their normal positions.

What Ed said. I might add that with "fake" POTS coming from a cable modem, even if the polarity is correct the modem will do strange things like causing all the LEDs to strobe and the modem to reboot or go off line. This, of course depends on the brand of modem.

-Hal