Canadian Provider

The number of simultaneous call that your provider can provide is totally up to your provider. Some allow just one, but many will allow multiple 'trunks', usually with an additional charge, so you would have to talk to your provider.

You could probably just forward your PSTN number ot your VoIP provider, depends on the phone company, sometimes there is a charge for this, call Bell.

If you do 'forward' your number to your VoIP provider, you'll still be incurring a monthly fee from your telco for that number. One possible avenue is for that provider to have that number transferred to their service. The montly service fee will be payable to them, but might be cheaper.
If you do forward, talk to your VoIP provider about the CallerID. You probably want to maintain the same callerid as the line you forward from?

Call waiting, ...again details that the provider would be better off giving you, and depends on the type of phone you'll be using at your location, but if they support multiple trunks, then it will ring the second line (if available) on yoru phone, and if no answer, goes to Asterisk voicemail. If they only allow 1 line, it may goto the voicemail system of your VoIP provider.

One way to set up VoIP is to keep the old landline in service, but get Call Forward Busy and set that to forward to your VoIP line. Many VoIP providers let you set your outgoing Caller ID to anything you want which allows you to have your outgoing VoIP calls showing up as your original non-voip number.

Now you'll have one incoming landline + one or more incoming voip lines. Generally you can place several outgoing calls as long as you're on a per-minute plan rather than flat rate.

I've set up a client to use them for outgoing service and they haven't complained, and actually preferred it to some alternatives. I can't speak for their incoming service, however, as I've never used it.

I have a Toronto based 647 and a few 800 numbers with Voicenetworks.

They are a tad expensive on the 647 DID vs. let's say, unlimitel. On the positive side their voice quality is always good.

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