So I learned an easy trick from a TAC engineer a while ago and I thought I would share it.
Like many others, part of my normal responsibilities involves routine testing of our ancillary and backup systems. That being the case, I am the lucky one who performs our SRST testing for our branch locations. Rather than having to use out of band management to shut down our WAN links to perform SRST testing, or driving onsite to disconnect the WAN links (we used to do this). TAC demonstrated a solution so easy, I was surprised we hadn't thought of it before.
Simply create static routes for the IPs of your CUCM servers to route to a Null interface on your router. This will blackhole the traffic that would be bound to the servers and the phones will register to the local router.
You should see your phones register to the router after a few minutes.
If you want to test your Dial-Peers after that, you can connect a phone local to you, cloning the settings of the branch location. I use a small router at my desk to create the null route and have my local phone register to the branch router. Then I simply place my test calls.
When your testing is done you can remove the null routes and reset the phones.