、。
If Jason happens to call, tell him I'm not in.
Literal
By-chance Jason [subject-が] telephoned-came-[conditional-たら], I [topic-は] not-here [quotative-と] say-[blunt request-ってくれ].
電話してくる ('to call up (toward the speaker)') combines する with the direction ~てくる. いないと言って is 'say that [I]'m not here.' くれ is the blunt imperative of くれる — rough, masculine, spoken to close peers.