I would try calling another vet in the area, you may be able to convince them to come out, or he or she may have the pager number or direct line to your vet.

If not, I would drive (or ask a horsey neighbor to go) to the vet's office. The phone may just be off the hook/ out of service. If nothing else, you/ the neighbor may be able to get some Banamine if they're on the line with another emergency.

Otherwise, trailer (borrow if you have to) to a vet. Sometimes just the stress and movement of the trailer helps get the gut moving.

Have you had a big change in your weather (from cool to warm, or big change in barometric pressure)?

Ride smarter, not harder.