我女友會出trip,之後會stay behind同埋我會過去join佢, 我想問如果大家都冇當地電話咭, 但係又要聯絡, 咁我係唔係照打番+852<mobile no>, 咁就ok呢

subject to your mobile service have activate roaming function or not.
Also subject to your mobile services provider have any 'agreement' to local country mobile services provider(s) for roaming services connection.
Further the dail number refer local mobile services provider requirement

假設有roaming, 唔理幾錢一分鐘...

呢個方法係唔係可行?

better to check with your and her mobile services provider, is your destination city/cities is within their roaming services providing list