To be fair, most comercial farmers, and that covers 999 out of 1000 would have dealt with a non high value (Read sire breeding) lambing issue themselves, and the rifle or the best efforts would be the case, and as such the vet would be just acting as normal. A ewe worth 60 avg (ignore the smallholder pricing and think auction) with 1 or 2 lambs, which are worth 0 until weaned, is not worth a vets call out.
As such make clear your animals are pets, or your prepared to pay more than the animal is worth.
Also remember, they may be busy, what if they have surgery on, short staffed or just a busy day, visiting you may simply be a choice of one sheep, (an unimportant meat animal) or several seriously ill dogs and cats. Travel time etc.