Sounds like a job for QoS instead.
1) Give your phone a static IP on the "Connection/DHCP" page.
2) On the "Firewall/QoS (Download)" page create another class called "Voip Phone". Don't bother with limiting the bandwidth yet.
3) Add a new rule: Source IP - the one you gave to your device. Set service class to "Voip Phone".
4) Start a call and see if the traffic shows up in the corresponding class. If so, take a note how much it actually uses and whether that sounds reasonable (and you didn't accidentally capture something else or nothing at all with your rule).
5) If everything seems to be in order, you can go back to editing your "Voip Phone" class and setting a Maximum Bandwidth limit generously over what you expect the phone to ever want to use.
6) If everything is still good, repeat steps 2-5 on your "Firewall/QoS (Upload)" page. Note that this time you want to set your rule using Destination IP.
Note that you will need a second class where everything else can fall into by default. Set no restrictions on this and set it as the "Default Service Class" on both your Upload and Download pages.
Hopefully you can set up total bandwidth and other option on these pages, if not, this is a good read to start with:
https://www.gargoyle-router.com/wiki/doku.php?id=qos