Considering Plusnet

[#1]

A question for the plusnet techies:
I have recently bought a business and have inherited a weird telephone and broadband setup:
We have at least two BT lines in the property (possibily more):
- Line 1: Telephone number (A), (B), (C) and (D). (A) is the primary number. (B), (C), D) are looped to (A) and are supposed to ring if (A) is busy.
- Line 2: Fax number (E). This also happens to have broadband on it.
Both lines are with BT and we receive separate bills for them.
I am quite pleased with plusnet broadband at home and am considering using it for business too.
My requirements are as follows:
1 - I need a single line
2 - I need telephone number (A) on that line.
3 - I need broadband on that line.
4 - Telephone number (A) needs to be permanently diverted to a different landline number (F). (F) is a dedicated reception at one of my other sites where calls are handled.
5 - (F) has the facility to take as many simultaneous calls as required. Please confirm what will happen if someone rings number (A) and whilst on the line another person rings number (A)? Will it come across as busy or will the call go through? In my experience with other providers the call forwarding works in a manner where simultaneous calls are possible if the number that the call is being forwarded to can take a second call but I just want to double check this with you guys.
What I am trying to achieve is not complex at all but given the existing setup it can be a bit tricky. I can't afford to loose the telephone number under any circumstances so I need some kind of assurance before I can take the leap.
Thank you

[#2]

If I were in your shoes I would port number A to a VOIP provider as this will give you ultimate flexibility in routing and also allow multiple simultaneous calls to the incoming number.
I'd then leave line E in place untouched...

[#3]

I've been trying to achieve that for the past three months but there are issues with porting the number to VOIP.
The line rental on the BT lines alone is £300 per month so I can't wait any longer and have decided to try this instead.
VOIP would have been ideal.

[#4]

If there are issues going to a VOIP provider I would have thought you'd have the same problems porting the number to Plusnet. If BT are playing games with you on number porting it is grounds for a complaint to Ofcom.
I've just been quoted £322/month for a 10/100 FTTP (Fibre to the Premises) contention 1:1 and just an extra £20/month for an additional 20 Meg outgoing. I can run a lot of high quality calls over that and get advantage of VOIP rates.  Note that normal domestic/business "Infinity" is shared bandwidth at the exchange so you'll get reduced real capacity at busy periods.
Have you examined the impact of call charges from BT?  Their standard rates per minute and call set up costs are painful and if you have a significant volume of outgoing calls the transfer to VOIP can subsidise the cost of direct fibre. You need to check how call charges are "rounded", is it to the nearest second or up to the nearest minute? Depending on call volumes/durations the impact can be substantial.