Recently I have received many questions on how to test SRST. There are multiple options that are available, with varying consequences to the rest of the topology. We'll go over a few of them below:
If you have phones that are directly connected to the router performing SRST BR1 (R2) or at least connected on the same LAN segment, the process is easier, and simply requires blocking WAN connectivity to the CUCM cluster. You could do this by either shutting down the WAN interface, or by creating a null route for traffic to the cluster (ip route 126.96.36.199 255.255.255.0 null 0) This route would send all traffic destined for the 188.8.131.52 /24 subnet to Null or in other words nowhere.
Now, if you have phones that are connected through a VPN connection that goes through the PSTN router, the data path does not go through R2, and therefore shutting down the WAN interface will not force the phones to drop registration from CUCM and re-home to the SRST router. You could then block traffic at HQ (R1), but all communication with Phones registered to CUCM, and Voicemail/Presence/CUCCX would then be broken. Instead, you can block packets through the PSTN router. Use the same ip route statement from above, and apply it to the PSTN router. Now the phones will register to BR1 (R2), and you can perform testing, including making calls back to the HQ site.
Don't forget to remove your IP route after performing testing! Otherwise, many other parts of your lab will not function! Perform a ping to the CUCM, to ensure you have removed the route, or brought up your relevant interfaces.
Hope this helps