From T-mobile site:
Keep your existing phone and you could get unlimited texts with one of our no-hassle just SIM plans.
Is just SIM for you? Here's what you get.
* Unlimited texts to ANY UK network with just SIM 20, 30 & 40
* Up to 1500 minutes to ANY UK network
* 30-day contract and you can keep your existing phone
Off-peak Inclusive minutes apply in the evenings (7pm - 7am) and all weekend, for local, national and call to any UK mobile network
Peak time calls cost 30p to an network.
Inclusive minutes are for calls to UK local & national numbers and any UK mobile network at any time of day. Voicemail retrieval is not included in your allowance.