I'll start by saying I have no direct connection to the company we use, other then the services they provide to us. I'm not directly advertising them just trying to give a fair review of the service we've had.
The company we use is numbergroup.com, based in Birmingham. They've provided us with multiple SIP trunks and several numbers. I did a lot of shopping around when looking for VOIP services and found several that would give you a number, but it tended to be 05 or even 08. I really wanted 'real' numbers and most of the offerings out there are rather expensive with monthly rental of such numbers.
Not so with this company. They have allowed us to have several numbers on a number of exchanges that route to the SIP trunks. To explain how it all started. We're a small community radio station. we wanted a number on a neighbouring exchange. We got that, set it up, then added more numbers on other local-ish exchanges, so we now cover about 14 exchanges with numbers that come to the one line. Because we can select from the available range we have picked out some really nice numbers too, at no extra charge.
Then I added my home line, which allows us free calls (between SIP trunks on the same provider), only I cannot dial out it seems. I've added other trunks to cover laptops that we take out for events, with the addition of mobile broadband we have a phone attached using a USB adapter and a softfone on-screen. Each laptop has two numbers, one on each local exchange.
What I'm trying to add next is a link over VOIP that will allow us to send streaming audio over this technology, that would be broadcasting live, using VOIP and mobile broadband, I'm still working on that as sound quality has not been good enough to broadcast so far.
The only drawback for most will be that you have to satisfy the operator that you are a company - we are, so no problem there. For the applications we're using it's all SIP to SIP for calls we make, and people calling us see a real number, not a Skype name or an obscure 05 VOIP number.
Cost is brilliant - nowt. That's the best part about it, apart from the good service they give if we have issues.
As mentioned, they are not free for all numbers and services. Business only I'm afraid. What we have is a quite sophisticated setup now. I hate to think what the cost would have been had we tried this with BT lines.
Such a pity that VOIP aad SIP is still a dark secret for many. I'm still getting my head around it all. Hope the info is of use to others.