As I said, there may not be an alternative, but no harm in trying, just don't go phoning people in the middle of the night.
Also, remember 141 withholds your number, not that I am encouraging calling people and hanging up.
Anyway, from the lists available on Ofcom's site
here, 0870 146 is operated by Energis. If we look at the operators in the Exeter code (01392), we find Energis have only one prefix, and that is 01392 89.