Skype VOIP phone set up for office, not working that well

We are trying to use a Skype phone system via our computers at our new office. We are doing it this way because we need to keep the initial and monthly cost as low as possible. It is just not working right now as sometimes the calls do not go through at all or you drop them.
We have skype with the Pretty May software as our PBX. The problem seems to be our internet speed. We have Qwest dsl 600/260 with clearwire 1500+/100 hooked up together via a duolinks sw24 syswan.
The phone quality is only good when few are on the computer. We have at most 10 people now that will be using the internet to check email and run quotes. We plan on having another 10 by the end of the year.

Here are some speed tests that I ran throughout the day:
1. 2500/168
2. 2000/35
3. 500/260
4. 576/208
5. 561/209
a. It seems according to the numbers that the clearwire was working on the first two tests but not the others, it has the fastest download 1500+ on average but poor upload.
We can buy a T1 line at 1500/1500 for $350 per month. Will this solve our problem and be enough for what we need or will it not? If we sign up for T1 we are locked into a three year contract. I would only want to do that if it worked.
PBX phone systems are pretty expensive and then we would have to pay around $35 per line.
Is there a better way?