Create two different IPSEC policies, using the same tunnel.
Create a Service Group and add your custom services to that group.
On the first IPSEC policy (we' ll call it the VOIP policy), do your normal source-destination rule, but include the custom service or the service group you created (if you have more than one port to prioritize). Then hit Advanced and turn on traffic shaping. Give the policy a GUARANTEED rate and then make your Maximum rate AT LEAST as much as the guaranteed rate. Set priority to HIGH.
On the second Policy, just do you normal source-destination rules here, but leave services to all - for all other traffic. Hit Advanced again and turn on traffic shaping. Set Guaranteed bandwidth to zero, and give it a maximum rate that is sufficient for your purpose but does not hog the entire pipe. Set traffic priority to medium.
With this setup, VOIP traffic will match both policies, but use the first one it sees. So set the VOIP policy first in the list. Other IPSEC-bound traffic won' t match those rules and will move on to the next policy.
Hope that helps - I have to do this with some firewalls on slower DSL pipes to guarantee service - just not VOIP - same principle, different ports.