Well you say you have a DSL connection with 10 mbit of bandwidth, but you don't specify what your "uplink" bandwidth is (I'm assuming you gave the downlink).
Remember that in this case the uplink bandwidth is the most important because you are not very concerned with what the third party says to you, just what you say to them.
The thing is, I also have a 10 mbit downlink... but I only have a 1 mbit uplink. That means I can only make 10% of the outbound calls I could make with a 10 mbit uplink! So, as you can see, it is very important to know what your uplink connection is rated at.
Also, the audio codec you use will also play a large role in bandwidth. GSM uses only about 5 kbps each way, but the more commonly used G.711 codec (higher quality) uses about 80 kbps!
Finally I just want to add that some users in the past have complained about a bug causing IVM to have difficulty with more than 3 concurrent voip calls, I don't know whether this bug has been squashed.