If so, which VSPs are people using and are they using callback primarily from landlines or mobiles?
I see that 2degrees states in their T&Cs:
(e) You may not make 0900 calls, collect calls, special number calls, or any calls where charges are not immediately deducted from your Pay Now Account, or use your Mobile Phone for voice over internet protocol (“VOIP”) or message over internet protocol (“MOIP”) communications.
(couldn't find similar conditions for Vodafone or XT) but I wonder if callback would qualify as collect calls, i.e. another party / account is paying for the call.