What causes dropped telephone calls and what can I do to address the causes?
Telephone calls can be dropped for a variety of reasons but if you're suffering regularly then there are few things you can check and remedy to resolve them.
- SIP Session Timers
- SIP ALG
- Talk Off
- VAD - Voice Activity Detection
- NAT Issues
SIP Session Timers
SIP session timers tell the SIP Servers that the calls is still alive as looking for a BYE message is not reliable. If a SIP Server thinks one end has hung up (i.e stops receiving KEEP-ALIVE messages) it will drop any open call legs.
SIP ALG
Read our guidance on SIP ALG HERE
Talk Off
Talk Off happens when phone tones (DTMF) are wrongly interpreted during a call. This is easily resolved by ensuring the recommended settings for DTMF transmission OUT-OF-BAND as per RFC2833 are enabled.
VAD - Voice Activity Detection
If in doubt, turn this off as it essentially saves bandwidth by not sending full audio if nobody is talking but not everyone can support it.
NAT Issues
SIP doesn't like NAT. It's that simple. Most people have to use it as IPv4 limitations dictate and few providers fully support IPv6.
