"Does dim rhaid i chi ffonio."
Translation:You do not have to phone.
does dim rhaid ... - there is no need...
rhaid i chi beidio â ... - 'you need to refrain from ...' which is better translated as 'you must not ...'
Thank you - that has now been put right in the course materials, although it may take a little while for the correction to appear in Duolingo.
I translated it as "You need not phone". I guess people just don't talk like that! :)