Oh, my phone card is about to run out... Well then, in 30 minutes.
Literal
Ah, already phone-card [subject-が] run-out... Then, 30-minutes later [at-に].
テレカ is clipped from テレホンカード 'telephone card' — prepaid cards for public payphones, once ubiquitous in Japan. なくなる 'to run out, to disappear.' The sentence dates itself — phone cards are a relic of pre-mobile Japan. じゃあ、３０分後に arranges the next call.