As mentioned by Orsonkart, you can use some other prefix operators like Call18185, 1899, 18866, etc whilst using SkyTalk because you are continuing to pay linerental to BT.
You can even dial 1280 and have any of your calls routed via BT which if you have BT's free Caller Display then you'll need to do this to continue to get the free caller display. If this is the case, I'd recommend during the evening & weekend to route 2 calls a month via BT (6 calls in a quarter) to continue to qualify for free caller display from BT.
Apparently sometime in the future SkyTalk plan on offering linerental as well which I would personally recommend
resisting moving your linerental away from BT because then you could lose access to other cheap prefix operators like Call18185, 1899, etc.
Remember, whilst your linerental is being paid to BT then BT have to allow access to other call providers which means you can always benefit from the cheapest providers and you are not stuck to a minimum term contract so can move as and when you want if other cheaper providers can beat Sky Talk.
If you move your linerental away from BT then you may lose access to other call providers and you may be stuck to a minimum term contract of generally 3months. Some telephone providers (currently namely TalkTalk) insist that you move your linerental to them as well as this means TalkTalk can prohibit/block access to other cheap providers and this appears to be what they are doing going from forum reports I've read.
You may want to consider getting an Orchid Dialler box which would allow you to automatically route certain type of calls to different providers at different times of the day to ensure you continue to get the cheapest possible prices. For example, you could setup the Orchid Dialler box to automatically route mobile calls via Call18185 instead of having to remember to dial the prefix first. You can also use the Orchid Dialler box to block access to certain number types saving you paying BT to do the same!
You can get an Orchid Dialler box from
here but if you read
this thread on MSE you can get an extra 20% discount when ordering online.