Iphone Pay As You Go Card

[outrageous]

My Sister who lives abroad has an Iphone 5s and wants a Pay as you go option here in the UK which she visits on a regular basis ... as I am on a contract I dont know ... I need one that can be topped up when needed and will last over a long period of time ( as top ups might be every 3-4 months ) also the best deal for Texts // Calls and of course roaming so she can you the Imessage whilst out and about....
Any help very mush appreciated Thanks in advance

[madiba]

I have two contracts with 3 mobile and also run an iPhone 4 on pay as you go. The PAYG sim doesn't loose its credit at the end of each month and as such is ideal for a phone that's used infrequently. Hope this helps

[madiba]

Sorry the PAYG sim is also 3 mobile

[outrageous]

Thank you appreciate it