I would question whether you actually need a Land Line number for your Small Business these days, unlike a few years back, just about everyone has a mobile and use there bundled mins for most calls. In fact most people are now more aware about ringing Land Line numbers from there mobile, namely 08XX and 09XX numbers.
To use a Land Line and then have all its calls diverted to your mobile, when probably 90% of them incoming calls are from the callers mobile, is just an additional expense burden on you and free money to the Telco Provider, been there, done that.
Another thing to consider is if your using a Land Line number, some confusion will a rise when you then call or Text from your Mobile, ie, 'Im Outside'.
Sounds like you need to retain your T Mobile service due to its package, totally understand that as I get the same deal with T Mobile for £15 pm, although I do use another long term Vodafone Number for work calls, only another £8 pm for reasons that suit me. Otherwise consider moving over to Orange or Vodafone mobile tariff with a Land Line Number attached to it but you wont get the same deal that your currently on, so more expense.
Stay away from Alpha Dial and Personal Number Companies that offer free Mobile Divert with Land Lines starting 070, 0844, 0845, 087 etc, as your customers wont thank you when they get the Bill from the Mobile or Phone Supplier.
Also a lot of Companies offer cheap Consumer VOIP Land Line Calls to your Mobile via an Phone App, problem with that is, it relies on good solid 3G Coverage and and a lot of missed calls when your in rural areas with 2G only Coverage.
If you do need to stick with T Mobile Al, why not ask them to supply you with a good Rememberal number (Silver or Gold Numbers) for a one off fee. Or find and choose another T Mobile number with your own Alpha Dial from a keyboard, ie example 07X 4 TAXI AL = 07X 4 8296 25.