I think that this might be exactly what you are looking for.
Shared shaper allowing 10mbps for VoIP and only 1mbps for other traffic.
https://docs.fortinet.com/document/fortigate/6.4.8/administration-guide/933502/shared-traffic-shaper
However it's not the only possible solution.
If you do use SDWAN then you can DSCP tag the traffic and then use SDWAN rules.
Examples of how to config are here: https://docs.fortinet.com/document/fortigate/6.4.8/administration-guide/504928/dscp-tag-based-traffic-steering-in-sd-wan
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