A PAYG punting phone costs as little as 20 to buy (with top-up) so why wouldn't you?
You never know when a WG might send you promotional texts at awkward times, harass you after a negative review, look you up on social network sites, or post your number on the internet. AW could get hacked, WG's phone could get stolen, etc. Even if you don't have any problems in your punting career, a punting phone still gives you peace of mind.
Ultimately, it's your choice what phone you will use for booking WGs.