Dependant on where you are trying to ring, this might work for you. I'll assume UK landlines.
Look at the various voip software, I use voipstunt so that will be the example.

Go to www.voipstunt.com and download their software.
Open an account with them by placing €11.60 (about £8.12p) of which €10 (£7) will be usable credit.
This will be valid for 120 days.
Set up the software so that your home number is in the "Direct callback nr" box.
In the box at the bottom enter your target number. Land only because mobiles are too much.
Press the green button.
Your landline will ring. Pick it up and you will hear a ringtone as it tries to connect you with the target.
Upon connection your account will be debited by €0.03  (2.1p) inc all taxes.
You can now talk for up to an hour before the system cuts you off. You can redial if you wish.
There are several countries, including their mobiles, that are free (+ connection fee) eg USA land & mobile.
Credit remains for 120 days and is lost if you do not top up within 5 days of termination. If you top up in time, all remaining credit is kept + the top up.

So you can connect for 2.1p doing it this way, or for free if you want to go through the computer as a voip call.

That's true about the 300 minutes. It is actually in a 7 day period, not a set week. Ie Sun to Sat. I know a week is 7 days, but they'll calculate it slightly differently.
But if you were to look at your 1899 bill, how much time do you actually spend on the phone during the week, and then how much at any one time on one call.
For most people, 5 hours of calls in 7 days is probably more than enough.
Also to bear in mind is that ALL foreign calls are chargeable with 1899, whereas a lot are included in your allocated time with voipstunt (and their sister companies).
If you do go over your 300 mins, then the call rate to the "free" numbers is then charged at €0.01 per min. But if you were that heavy a user, there is nothing to stop you opening another account and using the same software to log out as one and log on as another.

This is for a 7 day / 300 min period  using a BT line
If we use calling the UK as an example. The maths is based on using the full 300 mins (5 hours) and taking 5 calls to do it.
(1)    1899 to landline                          = £0.00 for the mins + £0.15 connection = £0.15
(2)    voipstunt using the pc to dial thru = £0.00   "-----"     + £0.00 connection  = £0.00
(3)    voipstunt using callback               =£0.00    "----"      + ( €0.15)  connection = £0.11  rounded up.
Voip wins

Now if we go to 10 hours and 10 calls
(1)  is now  £0.30
(2)  is now  £2.10  (300mins at €0.01 per min * £0.70 exchange rate)
(3)  is now  £2.31  (300mins at €0.01 per min * £0.70 exchange rate) + 21p connection.
In this example it is better to use  1899 if you going to exceed your time, but you can have multiple voip accounts


However it changes if you are using a  NON BT line  (Telewest, NTL XLN or whoever)
If we use calling the UK as an example. The maths is based on using the full 300 mins (5 hours) and taking 5 calls to do it.
(1)    1899 to landline                          = £3.00 for the mins + £0.15 connection = £0.15
(2)    voipstunt using the pc to dial thru = £0.00   "-----"     + £0.00 connection  = £0.00
(3)    voipstunt using callback               =£0.00    "----"      + ( €0.15)  connection = £0.11  rounded up.

Now if we go to 10 hours and 10 calls
(1)  is now  £6.30  (600mins * 1p per min + connections)
(2)  is now  £2.10  (300mins at €0.01 per min * £0.70 exchange rate)
(3)  is now  £2.31  (300mins at €0.01 per min * £0.70 exchange rate) + 21p connection.

In this example it is better to use the voip program. Here you have paid an extra £4 for the convenience of not having to put the phone down after an hour.
Obviously an example can be found that disproves the benefits shown, but you have to assess your actual needs first, then find the appropriate solution.
Are you able to provide some figures regarding your phone usage, inc mobiles, land and foreign. Perhaps those on differing systems may be able to provide a better solution for you. I guess you are on the BT network for you to get 1899 at the cost you do.