Since August 2009, 0870 numbers have not paid out revenue share. The security firm no longer has a financially advantageous reason for using these numbers. They may not be aware of this.

Presumably, your 0870 calls are inclusive during evenings and weekends but not during weekday daytimes. That's when you're likely to be paying somewhere between 5p to 10p/min plus a connection fee of around 15p per call. You don't say which telephone provider you are with. Whoever it is, the extra expense currently incurred on those 0870 calls would seemingly make it worthwhile for you to move to an AnyTime inclusive call package.

Using an 0844 number is very unlikely to be a useful solution. Those do not count towards inclusive allowances at any time of day, nor at weekends, as there is a Service Charge (somewhere between 1p and 7p/min) hidden within the call price and a connection fee of around 15p per call.

Ofcom's forthcoming reforms for 08 and 09 numbers will see the return of revenue sharing on 0870 numbers in June 2015, making these numbers work the same as other 087 numbers. They will no longer be inclusive calls.

In the long term, the security company should move to a geographic rate number with a set monthly fee for their services, or be more open about the charges incurred by using their system. Ofcom will shortly require businesses using 084, 087 and 09 numbers to declare the Service Charge incurred on these calls.