DP has been seeing DSD regularly after their estrangement of over a year and they are gradually getting to know each other again and rebuild their relationship. 
One of the constraints is DSD limited text allowance. DSD has a contract phone paid for by her mum, this is in lieu of pocket money, so she hand I cash.
DSD often admits to DP that she goes over her free text allowance and her mum has had a go at her because she's been hit with high additional charges on the contract.
I have suggested to DP that he asks DSD mum how much it would cost to change DSD contract to an unlimited text plan and if it's a reasonable amount, offer to pay the difference for the duration of the contract.
As I am the only income earner in the household, it would mean that I would (in effect) be paying for it, although DP and I don't see it like that. DSD mum will know that though, and it is likely that she will tell DSD.
The problem is that DSD refuses to accept me in DPs life still - they are rebuilding their relationship independently of me - and I am happy to prioritise the expenditure because it will increase contact between DP and DSD.