TraiTel Virtual Numbers are used to make calls between TraiTel users, without going outside the TraiTel network, and also to make calls to the PSTN.
Can people call me on my virtual numbers?
Yes! IF they are fellow TraiTel users. You can't dial Virtual Numbers from the PSTN, for that you need a Real Number.
Do I need to sign up to anything to use a virtual number to make outbound calls?
No! Every new TraiTel account gets a Virtual Number by default, and the number itself is free, you just need enough credit in your account to pay for the call you are making. Simple.