If this reads right, then if you run out of Xnet mins on O2, you could dial cheapchat, and use them to connect for a lower cost that dialling the mobile directly.
http://www.o2.co.uk/mobilestariffs/tariffs/specialnumbersNon Geographical Numbers
O2 Pay Monthly O2 Pay & Go
0500* As per standard UK Fixed line calls 15p/min
055 As per standard UK Fixed line calls 15p/min
056 As per standard UK Fixed line calls 15p/min
070 50p/min 50p/min
07744 25p/min† 25p/min
07755 25p/min† 25p/min
0800* As per standard UK Fixed line calls 15p/min
0808* As per standard UK Fixed line calls 15p/min
0820 As per standard UK Fixed line calls 15p/min
0844 As per standard UK Fixed line calls 15p/min
0845 As per standard UK Fixed line calls 15p/min
0870 As per standard UK Fixed line calls 15p/min
0871 35p/min 35p/min
090 / 091 Click here for information on Premium voice
118 Numbers Click here for information on Directory enquiries
Standard minimum call charge applies unless otherwise stated.
† 1 minute minimum call charge applies.
*O2 Callers usually have to dial the number without the leading ‘0’. There are some 0800 numbers, normally Telephone Helplines, which are free to call, for example: 0800 1111 (ChildLine), 0800 441111 (Bullying Helpline), 0800 400222 (ChildLine – Textphone), 0800 555111 (Crimestopper) and 0800 7383773 (0800 Reverse).