I've got a Gigaset N300 VoIP base unit attached, which was up till recently running fine. I'm not getting issues with not being able to hear people who I call and calls cutting out all together.
I got in touch with my VoIP service provider who said the issue is possibly because SIP ALG (application layer gateway) is enabled, which can corrupt the VoIP traffic packets. I was advised to disable it.
Looked in the HUB setting, there was no such setting. After doing some digging, found that the HUB 3 has this setting automatically enabled and is not possible to turn off. Many people are saying the only way around it is to put the HUB into modem mode and attach a Router that allows enabling / disabling of the SIP ALG setting.
I again went back to my VoIP supplier to ask if there is anything else I could try because I'm with Virgin with a HUB 3. I was informed I am probably running an older version 3 HUB, and that a newer version or a firmware update would allow this setting to be disabled.