There are a couple of choices here. Frame is good but expensive. Cable and VPNs work very well but the truth of the matter is I dont consider cable to have the uptime of Frame nor does commercial cable have the support structure in place for a true support of business level requirements.
Normally speaking a single router would do the frame and the ISDN. You would configure the ISDN to be the backup interface for the frame. It's possible to mux multiple ISDN links together to build bandwidth. It's also possible to run 2 frame circuits over different POPs and then load balance across them. This is can be very effective. Keep in mind that ISDN backups is not really intended to provide the same bandwidth as the primary link. If that is a business requirement then give serious thought to running a 2nd frame.
HSRP is for having multiple routers/switches look like a single router/switch to the network. Each has the same config and uses a single virtual MAC adress. They talk between themselves using multicast keep alives and when the primary router/switch drops off line, the standby will take over. This is somewhat wasteful of equipment so it's better to set up stand by *groups* and then each router can be used while still acting as a standby link via the group.
You can better advice with a better set of requirements. First thing is to do a network audit and build up the requirements. Then add 50% to what you think the users need

Once you have the user requirements in place, look at technologies that can meet those requirements. Sometimes you first get the budget numbers but I find it better to work out the wishlist first then apply the budget to it and see what can be adapted, kept or dropped. Many times the users will say they have to have X bandwidth as backup until you give them the dollars to do it. Then they freak... *cant you do it cheaper???*.. a smart engineer will already have plan B in place to show them. Half the time it's also an educational process to inform the users why things need to be done a certain way and what the tradeoffs are for each way.
Hardware can not be recommended until you have the traffic loads documented and business needs identified.
MikeS
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