This is an ongoing list of routers/modems known for their lack of VoIP reliability. If you would like to see a list of approved routers, please view the article called "Approved Routers / Quality of Service".
Cisco RV Series
These are not business-class firewalls - It is known to allow ghost calls (phone calls where there is nothing but dead air on the other end) despite having a NAT-based firewall. This cannot be fixed.
D-link is consumer-grade only. These are reported to stop audio transmission after 10 seconds of the call being connected. This cannot be fixed.
These are known to have issues with audio transit. Regardless if the SIP ALG is on or off (which requires additional licensing in the first place), audio will traverse inconsistently. This cannot be fixed.
These are ancient devices that do not have processor chipsets that are capable of handling VoIP. The technical issue is related to packet buffering and discards.
Netgear CG3000DCR and WPN824
Netgear products are generally excellent, except these two models, which has a SIP ALG that cannot be turned off (even by their technical support).
Intermittent audio loss. Per TWC engineers, the processor on this device is insufficient for realtime processing, and will cache packets in a temporary buffer if the queues get too full.
These use symmetrical NAT, preventing incoming calls from working. The built-in SIP ALG does illogical IP replacement. These routers MIGHT work with the SIP ALG disabled, but stay away.
These come with the SIP ALG enabled. This can be disabled, however these are reported to have up to 50% RTP (audio) loss under moderate-to-heavy load.
Symmetrical NAT enforcement makes for unstable inbound calling. The SIP ALG on these will re-write the IP address inside a REFER packet, blocking the ability to transfer calls.
Please note: A recent TP-Link firmware should have the ability to turn off the SIP ALG. If your TP-Link router can have the SIP ALG turned off, then your router is fine.
Verizon FiOS Quantum Gateway AC1750
These are forced onto Frontier and Verizon FiOS customers, unfortunately, but we still need this on the list because it has a SIP ALG forced on all the time.