If you have a mobile phone on pay-as-you go, you can request credits with a text message and the cost is deducted from your phone credit.
Two issues with this, however. Firstly it's twice as expensive, each credit costs £2, though this might not be a problem if you're on one of those "minimum £10 payment to get Internet data allowance" deals - in this case, if you're not using up £10 in calls each month, your AW credits are free, as you'd be wasting the unused phone credit otherwise.
Secondly, your phone has to be registered/verified with Adultwork, and you can't do this unless you are a registered adult with your phone company, and you can't do that with an unregistered anonymous simcard. But there is an advantage in having an "adult sim" anyhow (not necessarily the same number you use to contact WG's), as you can use it to confirm your nationality, which AW wants you to do for VAT reasons when you try to spend credits.
