Thanks for the replies.. I did a quick search in Sulit and found this (there may be more, but these are the ones that came near on top of the Sulit search):
"SIGNAL BOOSTER Repeater for Mobile phone cellphone GLOBE & SMART" ~ Php 5.5k-16.5k +
http://www.sulit.com.ph/index.php/view+classifieds/id/6843027/and there's a very-low-cost alternative:
Cellphone Wifi Signal Booster and Battery Life Enhancer ~ Php 250
http://www.sulit.com.ph/index.php/view+classifieds/id/6495694/The pricier one mounts on a roof or similar part of the house,
while the cheaper device seems to be sticked on the cellphone itself (undoubtedly, this will be weaker than the pricey alternative -- but if it helps a little, then maybe it could still be worth it).
Any comments on whether the 2nd item listed above might actually work? (so far, no feedbacks yet for that ad)