I don't touch 1571 - it is not always a great idea for it to take a call when the line is busy. For a start, if speaking to one of my sisters, we can be talking for 90 to 120 minutes (for 5p with 1899.com).
If you have 1571 enabled, and happen to have a long call (but don't use 'Call Waiting' to get a buzz to inform you someone else is calling) then the caller will have been charged 9.9p (?) connection and something per minute to leave a message, and get charged again if they ring back after 20 minutes or more and line still busy.
Then, unless things have changed, the only indication for the recipient is that the dial tone changes, to indicate some message(s) waiting. If you don't notice, does the system call you to tell you ?
For calling a mobile, if you don't have your own mobile on contract, it may be cheaper to use an Asda SIM (8p/min to landlines or mobiles) rather than some additional package with BT or Sky or some other firm.
MoneySavingExpert e-mail says Primus charges 8.99 (but most likely only on DD, and there's a chance it could go up - today TalkTalk has announced plan to increase monthly fee by 55p ).
If someone is sticking with BT, then with the planned line rental rise in October, they could save themselves about 30 pounds by signing up for the BT "Line Rental Saver" (probably need to sign up in September to gain benefit). From October, line rental goes up 50p with BT to 12.04 (monthly, payment by DD) but a reduced rate of the equivalent of 9.49 is available if you pay the full year in advance.
Clearly, paying out 113.88 may be a big chunk for many, but there's no way most people could earn interest of 30 pounds on that small an amount. You would be locked in with BT for the 12 months. Leaving BT would have a penalty as you'd lose the rest of the line rental that you had paid. It's perhaps their way of discouraging anyone from switching to another company like Sky, TalkTalk, or even Plusnet (owned by BT).