PRI Gateway - changing default gateway?

[JB-WH]

Hi.  First off, I should state I don't generally have anything to do with VOIP-specific stuff, and my understanding of it is shaky at best.

I've run into a situation where one of an organization's PRI gateways has an out-of-date default gateway. 

If it were any of the gear I normally manage, I'd just do:

route 0.0.0.0 0.0.0.0 ipv4.of.new.router
no route 0.0.0.0 0.0.0.0 ipv4.of.old.router

and call it a day.

This thing being a PRI gateway, I have no idea what, if anything, that will do to any existing calls.

Can I make this change transparently, or am I looking at someone having to deal with our typical irate users who ignore the maintenance window emails?

#sh hard
Cisco IOS XE Software, Version 03.15.03.S - Standard Support Release
Cisco IOS Software, ISR Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 15.5(2)S3, RELEASE SOFTWARE (fc2)
...
cisco ISR4331/K9 (1RU) processor with 1665730K/6147K bytes of memory.

Thanks for any help.

[Nithin Eluvathingal · Accepted solution]

I hope your requirement is just to change the default gateway of your Voice gateway. changing the gateway won't make any issues. 

Make sure after the default gateway change all the phone vlan and server Ip's  are reachable(bidirectional) from gateway.