Bell Canada → Re: free or low cost voip with caller id.

They do not provide your own, private PBX. However, many features of a PBX are there. You can create sub accounts (either SIP or IAX2). And you can assign internal extensions to these sub accounts if you want to.
You can choose where your DID(s) rings. For most people it will ring their main SIP account. However you could also have a ring group, or an IVR (phone menu).

For the initial startup you usually only need a voip phone (either softphone, ATA, hard SIP phone). You must deposit $25 which will be used as pre-paid money.

This $ 25 is some type of security money that one has to deposit or is it an advance payment for the services? The calling cost will be apart from this or is it included?

Advance payment, account balance that you chip away at whilst using the services.

I configured my account to send me an email whenever I get lower than $X dollars so I can top up via PayPal.

The best bang I've ever gotten for my buck, ever.

Mike