VLAN and VOIP traffic

Experts,

I am trying to work out VOIP quality problem in one of our remote locations.  Users complain of calls being choppy or the users cannot hear the party calling them.  I am sure I need the segment my network or enable some sort of QOS, but I am having problems working out what needs to be done exactly in order to resolve this problem.  

Currently, we have a Trixbox server that has POTS lines plugged into it on our LAN.  We then have Aastra phones in each office and then the computer is plugged into the phone.  In some instances there is a switch in the employees office and then the phone and computer are plugged into the switch since I only have one line that runs from the server room to the employee's office.

Network Infrastructure:

Primary Internet             Backup Internet
                                |
                                |
                         ASA 5505
                                |
                                |
             Netgear   GS748TPS Switch
                                |
                                |
           Computers, Phones, Switches

I can't use the VOICE Vlan on the switch since the computers are plugged into the phones and I don't think I can use another VLAN because that would cause the phone to have another IP Address and thus would cause the computer not to be able to access the network.  

Can someone please tell me what can be done in this network configuration to properly segregate my traffic so that I can improve voip call quality on the network?

Thanks!