You can failover to a second interface and ISP but you cannot simultaneously have two active interfaces, both with equal cost default routes, on an ASA.
To do the failover, you could bind your certificate/ssl-trustpoint to both outside interfaces isp1 and isp2. You would have to have some route tracking using ip sla to flip your default route from the primary isp to the backup in the event of a failure and some way of changing your DNS automatically to use the new outside interface IP instead of the old one.
To do what you're describing well, you would instead use a router with connections to your ISPs. You would have either Provider-Independent address or the secondary ISP would agree to route the address assigned by the primary ISP. In either case, it would have to be a routable /24 (or larger). The ASA sits behind the router and only has its single public IP address and interface. No mater which path the phones come in via, they get to that same ASA address.