If it is attention that's causing the problem in your home the only way this idea has the possibility to work is if you control every aspect of it.
All attempted negotiations (unless it feels right at a much later date) need to be blanked out and appear un registered.
A telephone sweet Jar or box - in view but 'ish out of reach.. giving out or offering a sweet is at your sole discretion .. this can be immediately the phone rings, during the conversation or with a hug and big well done at the end of the call.
She is little to be given complicated reasons as to why she isn't getting a sweet should & when she does swear.. so personally acknowledge & again unemotionally blank that behaviour, then .. at the end of the call calmly, happily & regardlessly give her a sweet with a 'thankyou for waiting' ... for the phone call to finish - before you were able, allowed or had the time to reward her - ie: you got through the call without having to verbally/facially/ emotively deal with any behavioural issues
... that is the part, that could backfire
lol, it may resort to you having to put a door between the two of you so that you can deny you heard anything - when the other children tell you .. give them a smile & hug and ask them to 'wink' bear with you both, don't keep this from the little one though, if she understands something is going on and enquires about it she may well be ready for more indepth reasoning as to why swearing during phone calls isn't necessary.. involving the others in your care also helps shift some of the strategy's responsibility & inclusion .. so should be done with care. They too, should without question be included / made aware that they are welcome to have a sweet when things go - or maybe not - according to plan ..
For tea time, as suggested, where 'dealing with' rather than the shock of a sudden outbusrt/slip up happened, I'd probably ride over and ignore the words as well, substituting for an alternative without acknowledging who spoke the misdemeanour
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Hope this helps .. let us know what happens !!