£17.50 a month for 18 months gets you 750 mins and 150 texts if you choose the right phone e.g. LG U300.
Quidco website shows that you get £100 from them if you go through their link - http://www.quidco.com/3-mobile
As far as I can see the phones that offer the whole 18 months at half price are:
LG U300 - Black
LG U300 - Orange
LG U890 Silver/Pink
Motorola V3X - Refurb
Sony Ericsson K608i - Refurb
Sony Ericsson K610i - Refurb
Nokia 6280 - Refurb
Nokia 6233 Refurb
No Cashback Hassles - Hooray!
£315 for 18 months less £100 quidco so -
£215 over 18 months is only is only £11.95 a month