Is the T-1 a data T? What are you using for phone lines?
Each phone takes about 80-110 kbps, so for the purposes of easy math, all 5 phones would use about 500kbps, or about a third of your T1.
I don't know what kind of data you are passing, if you are steaming the presidential debates or stuff on youtube.
The problem could be that your phone calls have to fight with everyone else for bandwith. It could help a lot if you had a router that supported QOS. Once you hit the Internet there is no QOS, but you need to prioritize the voice traffic out to the WAN.
Are all 5 users at different locations? Sometimes it can be the internet provider. I usually start troubleshooting by using a continuous PING from end point to end poing. Delays should be under 200ms, or else you've have problems with quality.
You can ask your vendow if they have the latest firmware on all the equipment.
