Perhaps a silly question, but have you subscribed to the 18185 service, or merely using the number?

I understand the 9 because your system has now been set up to be similar to a PBX sysstem. You can divert incoming calls all around the office.
However, and perhaps someone can correct me, is the divert not actually set up at the exchange? You dialing the *21* is a command to the exchange to activate the appropriate service.
So that all numbers leave the exchange and head towards your predesignated number? If that is the case, then you should not need the 9, because the call has not made it to your phone.
It is only a theory,but have you tried it?

CDM
I recently wrote a piece in one forum regarding a box of tricks that would do well in this circumstance. But because it was long and contained much information I don't want to write it again. So I trying to find the post so I can direct you there.
In the meantime, how long are your calls that have been diverted to you? And who is the provider of your receiving mobile? These answers could help in factoring the best reply.

If anyone else can remember the conversation regarding a newbie who wanted calls forwarded to his personal number at the callers expense, please enlighten as to where the heck that posting was. It was about 400 words long, lots of maths, involved a free line rental and a box from Burnside Telecom. It would be ideal for here.
Please paste the forum address onto this page. Thanks

I've pulled up a bit of blurb that I wrote for a friend who was looking at going to "feature line". So here's the info thaat would benefit you if you weren't aware of it.

Phone charges
To landlines it's 2.7p per min to a maximum charge of 10p, for a max time of 60 mins. (60 mins for 10p possible). Normal charges after 60min

To mobiles it's 10p per min to a max of 25p. for a max time of 60 mins. (60 mins for 25p possible). Normal charges after 60min

Be grateful you don't have a "3" phone as the charges would upto 18.3p per min, for every min.

ALL prices plus vat.

======================
18185
Weekdays to mobiles 5p per min, ALL mins.  Landlines  free.
Weekends to mobs 2p per min, ALL mins.   Land=free.

As you're a business, prices from here on are pence per min EX VAT.

mins    bt     18185
1        10     4.3
2        20     8.6
3        25     12.9
4        25     17.2
5        25     21.5
6        25     25.8   
7        25     30.1
8        25     34.4
9        25     38.7
10      25      43
etc

As you can see, it would be worthwhile looking at the average length of time that you have diverted. If the average is less than 5 mins then it is just worthwhile messing around. Over 5 mins, then I would suggest you stay with BT.
My lovely posting, that I'd spent a good amount of time and effort on, would have been wasted here because of your price plan.
This info is correct as of 16-05-06. I know because I spent half an hour chatting to BT.
You would only really gain on the weekend. But then you take their number and call them back using inclusive mins.


UPDATE
You can thank Dave for finding this. Whilst I said it wasn't relevant to you, it's still worth a read.
www.saynoto0870.com/cgi-bin/forum/YaBB.cgi?num=1144139641/13#13