We used to have an 0845 number when they were charged fairly at local rate wherever the call came from in the UK but turned out to be expensive to ring from a mobile. As you will know that has all altered now so we have changed to 0333 4567 021 so that all calls made from either a landline or a mobile are either included in the calling phones call charge bundle or at the very worst cost only the same as ringing a local landline.
I really don't think that a business customer will think twice about ringing a local rate number as opposed to an 0800 number.
We have a permanent divert set up at the exchange which goes to my personal mobile after 6 rings without getting answered in the office (should be a very rare occurrence). We have 4 ISDN phone lines including a dedicated FAX line plus 1 analogue line with the broadband. For outgoing calls we use a Premicell which uses the mobile networks where possible.
The 0333 number costs us £10 per month with no other charges and I can divert it to any other UK landline number free of charge within 30 seconds in the unlikely event of a telephone outage at our office.
The last thing a customer wants to hear when trying to book an urgent job is an engaged tone, or worse still an answerphone message