I don't often post on this list (which is why I'm replying to the digest - apologies if we've moved on since then), but I just wanted to comment on a few things in this thread.
@Richard - when you talk about SIP trunks, it's probably best if you clarify whether you're looking for a 'retail' solution e.g. something you can use yourself in an office environment for a PBX, FS box, etc or resell to clients that want to do the same) or wholesale - whereby you're going to be aggregating traffic from multiple customers via SIP trunks, hosted PBX, etc etc. There'll be a few differences both technically and commercially between these two offerings - e.g. most wholesale providers will only deliver inbound traffic to a SIP URI, and will more often than not expect traffic to originate from one or more static IP addresses.
@Cal not sure what AQL ratecard you're looking at that charges 26ppm for UK Mobile Orange, I'd hazard a guess you need to divide that by 10? Even when you do that's still a little on the expensive side for wholesale.
Wholesale providers charge minimums as they are, as the name suggests, wholesale. They are high volume low margin operations, and often don't sell 'retail' services direct to the public - e.g. Magrathea, Simwood - and will (last time I checked) require you to sign a declaration that you qualify to be a wholesale customer.
Gamma, BT, Kingston etc obviously do have retail offerings but also have separate wholesale divisions. Gamma and BT (and I'd imagine Kingston too) will also provide you retail services that you can white label and sell on to your customers under your own brand - no equipment required.
Companies like Gradwell, AQL (who jointly own a third party called Telephony Services, who you might have come across), SureVoip, SipGate are (in my humblest opinion) retail providers. They may also have white label offerings like the above.
Now... if after all that you fit into the wholesale customer category, you might want to split your requirements into UK and international, and again split inbound (numbering) from outbound. Why? Because as (I believe) Andrew said, there's no one service that'll meet all your needs.
Example: Voxbone will happily provide you numbers in umpteen countries, but you'll find that they're i) expensive for UK numbering (actually they're expensive full stop, but that's just my personal opinion), ii) there is a minimum spend, and iii) they won't help you much with your outbound requirements (except for 999/112/911 etc traffic in a few countries). Likewise, Magrathea and Simwood wouldn't be a bad choice for UK numbers, but if you want numbering in the US, Australia, etc, they're probably not going to be able to help.
As for BT IP Exchange, let's leave that discussion for another day, but suffice to say it's not a regulated interconnect product - so comparing a provider who hosts their numbering on BT IPX with one that leverages TDM connectivity to BT is like comparing apples to oranges both technically and commercially.
I'm not going to tag on a plug for my company to this thread as it'd be bad etiquette - but ping me a mail off-list if you want to chat further.
Cheers,
Alex