You want to lock down your firewall to allow specific IP addresses for your TEAMS-VOIP traffic. This is not recommended as our IP Addresses can and will change as required as the service expands. However you can check the existing addresses and modify your Firewall settings if required.
You can establish the exact addresses by running the following commands depending on which platform you are on:
nslookup -type=txt allips."your providers domain".xx
dig +short -t TXT allips."your providers domain".xx
These commands will produce a list of addresses you will need to enter into your Firewall whitelist table.
You can also use MXToolbox to look these up. You need to be looking for DNS TXT records for allips."your providers domain".xx