UniFi phone problem, Intermittent receiving calls

Broadband provider
ISP/package: UniFi VIP 5
Location: Bandar Sunway
Have you called your ISP's helpline?: Yes

Personal equipment
Modem: TM provided Huawei Home Gateway HG655m (previously was HG655a)
Router: TM provided L7 Networks L7-N-R2000 (with dual WiFi antenna)
Firewall used: ?

Problem details
First occurence of problem: June 8, 2013
Problem frequency: Intermittent
Problem description: Sometimes can receive in-coming phone calls. Sometimes cannot receive in-coming phone calls. No problems at all for out-going phone calls. Also, no problems at all with connecting to the Internet/WWW, Web surfing, and HyppTV services.

TM UniFi technicians have visited my home twice - during the first visit, they told me my line was OK and their network had no problems, but they changed the Home Gateway model HG655a to a newer HG655m. However, the intermittent phone problem continued. The second visit, they changed the external (roadside Telekom box) Port to another Port and asked me to monitor to see whether the problem still exist.

Unfortunately, yes, the intermittent phone problem still exists. Hence, this posting to see whether any one else is experiencing such a problem.