Well..
You can do one thing..
You can set smoothwall to use 128 kbps of bandwidth only , so that way the remaining is free for your VOIP phones.
Do do so you have to use delay pools !
I think smoothwall squid is compiled with support to delay pools , so all is required is to define proper delay pools.
Sorry this time i can't post the config required by you as Have not used them since long so can't exactly remember the syntax also i m in hurry.
PLease look thoroughly my few earlier post about delay pools in the forum, I remember i have give solution to problem like many times..
If you can write right delay pools then post it here, i will reactify if any problem in that.