> My bandwidth is very very limited. Not more than 140 Kbps on both

quoted text
> sides at any time. I use G729 as a codec in order to reduce

> consumption. Use the pf.conf below, when VoIP is the only traffic,

> the quality of the calls is excelent with no voice cutting at all.

> Now if I start a download I immediatelly see the quality degrade.

>

> That is why I thought of using some radical policy.

That's strange; it may be your connection struggles at much lower bandwidths than nominal - for instance, perhaps it suffers high packet loss  at 80% utilization; TCP could recover, but VoIP might be affected.
Doing what you want should be quite simple, though. There are many ways I can think of of detecting VoIP traffic if your ruleset manages to - have pf log (all) on a pflog interface dedicated to it, look at queue traffic - and many ways of blocking everything other than that. I can't think of an elegant way of doing what you want, though!