Free weekend calls are available on Flext 35 and above plans. I've just had a Nokia N95 on Flext 35 with Web n Walk Plus (as I use this as my broadband at home). Total package cost: £9.99 one-off plus £5.95 next day delivery charge. Monthly costs: £30.00 call package + £12.50 Web n Walk Plus. Seems a lot but this includes my broadband costs, free weekend calls to all UK mobiles and landlines, and £180 allowance on texts, picture messages, and weekday calls.
I've been with T-mobile for 18 months, and this is an upgrade for me, but T-mobile simply matched their new customer offer equally as is on their website today. It came at the right time for me, and may be the right deal for others too, so thought it was worth posting.