UK SMS Reverse Billing Using API

Who would you recommend?

[johnser]

1 - We're building a site at present and want to offer a free demo on it.

2 - Someone enters their (UK) mobile number on the demo and we send them an SMS via the sending companys' API.

3 - When they reply by text, we then want to send them a second SMS (which is free on the demo)

However, once they sign up for this service we want to charge them £5 for the second message they will receive using reverse billing in the future.

We're looking at clickatell.com for this but not sure they offer everything we need.

Has anyone done anything similar, who have you used, what is their delivery performance like, are they cheap but reliable?

All thoughts welcome
TIA
J