Although the Child Maintenance Options helpline -- run by G4S -- is an 0800 number and free to phone from landlines, mobile calls can cost up to 41p per minute.
Now the Department for Work and Pensions says it has struck a deal with two providers, 'Virgin' and '3', to remove the charges from next month, with the taxpayer picking up the bill.
Discussions with the other major mobile companies are "ongoing".
Separated parents have to phone the number in order to make an application for support using the new Child Maintenance Service.
Gingerbread, which campaigns for single parents, called the move a U-turn and "an important victory for families for whom every penny counts".
The industry regulator, Ofcom, has introduced new rules which will make calls to 0800 numbers free for all, including mobile users, but these will not come into force until June 2015.