Do you have a shop alarm system with a phone line connection not connected via a filter?
Every 15 or so mins, it connects, pulls the line down.
I had a neighbour with a home alarm system doing something similar... took me a couple of hours to narrow it down.
Failing that, make a local (free??) call on the line for 20-25mins just after a failure and listen for any 'obvious' noise or clicks. Won't be any good if it's noise outside audible, but you should be able to detect something.
15mins is too long a time constant to be a capacitive type effect...
Look for all things in the shop that are on timers or cyclic.
(Someone suggested a freezer) but that wouldn't be perfectly 15mins on/off (at least one would hope not).
Definitely an interesting one... rather you than me !!!
You'll be chuffed when you find it.