Track source on the telephone? (5)
I believe the answer is:
route
'track' is the definition.
(I've seen this before)
'source on the telephone?' is the wordplay.
'source' becomes 'root' (both can mean the origin of something).
'on the telephone?' indicates a 'sounds like' (homophone) clue (how it might sound over the phone).
'root' sounds like 'ROUTE'.
(Other definitions for route that I've seen before include "Established line of travel" , "Way of getting from A to B" , "The way to get somewhere" , "Way, line" , "Way, path" .)