You need a floating static route for the ISDN interface; something like "ip route 0.0.0.0 0.0.0.0 184.108.40.206 200", where 220.127.116.11 is the 2620 ISDN address. If you are using unnumbered on the ISDN interface, then you also need to add "ip route 18.104.22.168 255.255.255.255 dialer 1", where Dialer1 is your dialer profile. Once the serial link goes down and 22.214.171.124 is removed from the routing table, this new default route will be installed.
At HQ, for normal routing, you need to have a route for the LAN (or other networks) of the 1720; so something like "ip route 172.16.0.0 255.255.0.0 126.96.36.199". You would do this for each remote site.