Packet 8 Virtual Office Phones, Delay making outbound calls

Hello,
I have a Soekris Net4801 running M0n0wall 1.3b16. The internet connection is
Comcast Cable 20Mb/2Mb with a dynamic IP that changes infrequently. I am not
currently doing any traffic shaping. There are no tunnels, and the load is
quite light most of the time.

One of the doctors in this office just got 3 VoIP phones from Packet 8
Virtual Office (I think they are SIP). The phones work fine except for a 5
second delay on outbound calls (5 seconds of nothing between dialing the
number and a actually connecting the call.

 The doctor took one of the phones home with him and experienced no delay
when plugged into his linksys router.

 I spent an hour on the phone with Packet 8 support. The tech (who never
heard of M0n0wall) had me NAT certain ports (UDP 3478 - 3480 and UDP
5199) to one of the phones, which didn't help.

Does anyone have any ideas, or experience with Packet 8 Virtual Office?
Should I give the shaper a try?

Thanks,
Jim