Until we speak again, take care!
'Finché' — fin-KE. Stress on the second syllable. The accent is important — 'finché' (until) vs 'finché' (as long as) depending on the verb mood.
Use at the end of a phone call or message when there will be a gap before the next contact — a tender way to close that also sends a care message for the interval.
'Finché non ci risentiremo' (until we speak again) uses the future tense 'risentiremo'. 'Stammi bene' (take care of yourself / stay well for me) as a farewell wish is personalized by the 'mi'. The phrase bridges the gap between now and the next contact.
Nel frattempo, stai bene.
In the meantime, be well.
'Nel frattempo' (in the meantime) — acknowledges the gap before the next contact
Nell'attesa, prenditi cura.
While waiting, take care of yourself.
'Nell'attesa' (in the waiting) — the speaker is waiting for the next contact
Da qui a quando ci sentiamo, abbi cura di te.
From now until we speak, take care of yourself.
Most elaborate — covers the full time interval explicitly
Italian phone conversations, particularly between family and close friends, end with layered goodbyes — 'un abbraccio', 'tanti baci', 'ciao', 'ciao ciao', 'ciao!'. The farewell is never one word — it's a ritual of diminishing warmth signals that gradually bring the conversation to a close.