I use there 3g service never had any problems with dropped calls.
Although their phrase unlimited calls is little misleading, i.e 1968 "for 38hkd get unlimited idd calls." Means you can make idd calls but they come out of your price plan for certain countries and for others you pay at reduced rate, they are not free. But you can make as many as you like
Even their customer service person had to go and find out when I asked what it meant. Took two days to get back to me.
In uk I had unlimited local calls on ntl which meant free calls.
Signal is fine though never had a problem.