It's not a valid regex for a telephone number: what it matches is literally a single numeric digit followed by the text "{5-15}" and nothing else.
What they probably meant to say was:
^[0-9]{5,15}$
Which matches "any string of numeric digits between 5 and 15 characters long, inclusive."
To be honest, if that's the quality of their regex testing, I wouldn't give any personal information to them, and definitely nothing that contained a credit card number.