i use world pay zinc.
Works with mobile phone, I use it linked to my wifi tablet. it does have an online portal so you can take payment over the phone and you can give people to option to pay by secure link ( great for buying vouchers )
no monthly rental and if you look around you might get a deal on the handset ( £49- 99 )
It's ok BUT.
I hate paying 2.75% card fees!!!
Most other pay as you go solutions seem to be 2.75%, some give away free card readers and may or may not have the option for phone payments.
Sum-up have just started doing a deal of 1.9% with a free basic card reader, I'm hoping the others may follow suit.
The european parliament have just voted on capping charges for taking card payments...not sure if it will make a difference but I'm going to keep and eye out for some decent rates appearing!!!