Freephone numbers starting with 0800 or 0808 are now free to call from mobile phones as well as landlines.
The move is part of an overhaul of phone numbers and charges planned for many months by regulator Ofcom which has now come into force.
It said the cost of so-called service numbers, starting with 084, 087, 118 or 09, had also been simplified.
But there are warnings of confusion over the exact cost of these calls and that the burden on firms may rise.
The changes, first announced more than a year ago, will affect 175 million phone numbers.
- Mobile phone users will no longer be charged for freephone services
- The cost of calls to service numbers will be explained as "calls cost x pence per minute, plus your phone company's access charge"
This has prompted many businesses and organisations to move from 08 numbers to cheaper 03 numbers.
Ofcom said that UK consumers spent a total of £900m a year on 250 million calls to 084, 087, 09, or 118 service lines.
Calls to 03 numbers cost no more than calls to geographic 01 and 02 numbers.
David Hickson, of the Fair Telecoms Campaign, described the changes as "terrific news".
"The end of the rip-off numbers is not far off," he said.
The cost of calls to remaining service numbers has been split into an "access" charge going to the telecoms provider and a "service" charge set by the organisation or business being called.
But Richard Neudegg, of price comparison website Uswitch, claimed that these access charges ranged from 15p to 44p across the various mobile networks.
"People could easily be caught out if they are not clued up about their provider's new charges," he said.
He said this could be a "big worry", but Ofcom has suggested that, in a competitive market, prices could start to fall.
The total cost of a call to a service number consists of the access charge, plus the service charge (up to 13p per minute).
All this is likely to mean the mobile networks take a financial hit.
Various groups have also suggested that the number of calls to 0800 numbers could rise, as people might be more likely to make spontaneous calls to truly freephone numbers.
This could also add an additional cost burden to businesses and put more pressure on staff, according to Justin Hamilton-Martin, chief executive of Ultracomms.