One's phone rings, getting ready for action (8)
I believe the answer is:
mobilise
'ready for action' is the definition.
(as in mobilising or readying troops)
'one's phone rings' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'phone' becomes 'mobile phone' (mobile phone).
'rings' means one lot of letters goes inside another (some letters are ringed or contained by others).
'is' inserted into 'mobile' is 'MOBILISE'.
'getting' is the link.
(Other definitions for mobilise that I've seen before include "prepare for active service" , "Prepare for war" , "Get ready for war" , "rally" , "18d [GET READY]" .)