When saying a phone number, I say:
One, two three, triple seven.
Can I say also say that:
One, two three, quadruple seven?
One, two three, quintuple seven?
Maybe it's not important that it be a phone number, but in the US, we have a group of three, called the area code, then a group of three, and then a group of four. That means we'd never say more than four numbers at time.
202-327-7771 - Even though there are four 7s in a row, you keep that first three together. Two-oh-two -- pause-- three two seven -- pause -- triple-seven (if you want) one.
I may say triple seven or seven-seven-seven, but I would be very unlikely to say "quad seven" even if the number were 202-327-7777.
Because of the arrangement of the phone number, I would never EVER say "quintuple seven" because the first seven goes with a different set of numbers.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
In the UK you're more likely to hear
123-777, One, two, three, treble seven
123-7777, ... double seven, double seven
123-77777 ....seven, double seven, double seven, or perhaps, double seven, double seven, seven
I agree with Richard, except for the last variation:
which is best avoided, as the two adjacent sevens could confuse.double seven, double seven, seven