stress testing to new phone voice ip phone sys

How much bandwidth do you have from your ISP? And are you using either VLANs or QoS, and are you going with an onsite PBX or cloud based?

And here is a site where you can try testing for up to 50 calls simultaneously for up to 5 minutes: https://www.ringcentral.com/support/qos.html

Will you have a local VoIP box to which your phones connect or are they connecting to an off-site Central Office?

what bandwidth do you have, and how many simultaneous calls?  Do you also know the codec. We can do some simple math but many factors come into play.

"switch to voice over ip phone system"
It is critical here to know what you mean by that.  There are two sides to the phone system: how the phones connect and how you get service.  This results in three scenarios:
1)  Phones use VoIP to a PBX which gets phone service by some means
2)  Phones use non-VoIP lines to a PBX which gets service by VoIP through your internet connection
3)  Phones use VoIP to connect to a remote PBX through your internet connection

I've seen all three scenarios with my clients

With scenario 1, your internet connection doesn't matter where your local network does
With 2, your internet connection matters but your LAN does
With 3, both your internet connection and your LAN matter

Knowing which of the three it is will help in deciding if your infrastructure is adequate