、。
If he calls while I'm out, I'll call him back from my end.
Literal
By-chance my absence-while [at-に] he [subject-が] telephone-call-[came-かけてきたら], back [adverb-折り返し] I's side [from-から] telephone-call.
Three things stand out. 留守中 ('during absence') is built from 留守 ('being away from home') + 中 ('during'); に marks the time span. 折り返し ('immediately, by return') is an adverb for responding without delay, common in phone contexts. 私の方から ('from my side') uses 方 ('direction, side') to specify which end of the exchange the call originates from.