The Fair Telecoms Campaign group called it one of the worst examples of the misuse of premium rate numbers and criticised the charity for failing to comply with a requirement to disclose the cost of calls.
The group says references to the 0845 number should have included the statement: "Calls cost 3p per minute plus your phone company’s access charge."
This access charge is typically 10p a minute from landlines and up to 45p a minute from mobiles.
The NSPCC campaign invites supporters to make a suggested donation of £5 in return for a personalised letter from Santa to a child.
A spokesman for the charity said: "We are really grateful to everyone who has taken the trouble to order a Letter From Santa this year and very sorry for any upset that’s been caused."
The charity has changed the number so that callers are charged their phone company’s local rate.
Using a premium rate number is likely to have appealed to the charity because it would have received 3p a minute itself, unlike an 03 number, according to David Hickson, spokesman for the Fair Telecoms Campaign.
Hickson praised the charity for "doing the right thing" and changing the number.
He said he had contacted the media about the story after raising the issue with the charity and receiving "a dumb reply effectively saying ‘bugger off, we know what we’re doing’".