Vashi telegraph office, near vashi depot AFAIK. In Ddadr it's dadar telegraph office, Near Dadar TT circle, in front of Pune Asiad Bust stand.
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Taken long back from BSNL Maharashtra website, but most of the telephone numbers have changed.
LOCATION Tel. No. FAX NO. WOKING HOURS
ANDHERI (E) 26840555/26830717 26838340 1000 hrs to 1730 hrs
BYCULLA 23075590 23072867 1000 hrs to 1730 hrs
CHEMBUR 25283627/25282444 25272207 1000 hrs to 1730 hrs
DADAR 24182691/24111202 24135638 1000 hrs to 1730 hrs
GOREGAON 28724905/28740157 28725164 1000 hrs to 1730 hrs
KHAR 26483808/26485953 26460662 1000 hrs to 1730 hrs
SANTACRUZ (E) 26128070/26126966 26114473 1000 hrs to 1730 hrs
SANTACRUZ (W) 26603303/26603303 26604691 1000 hrs to 1730 hrs
THANE 25474858 25473002 1000 hrs to 1730 hrs
VASHI 27821685/27821700 27821882 1000 hrs to 1730 hrs
PAREL 24133741/24132344 24145054 1000 hrs to 1730 hrs