4 telco i had been use before,(within 1 year)
My experience,
Maxis sometime will drop call at outside,and good in office or in house(not my house and office only).

Digi less drop call at outside,but drop call too often in house and office(not my house and office only),even i pass road below highway bridge!

Celcom is good coverage and stable signal in kampong area,in klang valley,so far so good.But the customer service really sucks!

Umobile-sometime will drop call when my phone network setting using 3G mode(3G not stable,keep changing 2G/3G always.

conclusion,in malaysia,no perfect telco!!