The best way is to suggest to your provider that they look into deploying SRV records and point your trunk to a hostname that they would provide. This allows the provider to control redundancy, priority and load balancing and change it on the fly including adding and removing IP addresses. I would question the clue level of a provider that wants its customers to statically configure five different IP addresses on the customer hardware.
If that is unsuccessful, you could create five individual voice trunks and then on the grouped-trunk level either create five grouped-trunks with different costs to each IP in the order you choose, or group all trunks in the same grouped-trunk depending on whether you want a failover or load-balancing scenario.
