The way that VoIP traffic works is that it sends the relevant information (called packets) via your internet connection to you and your caller (and vice versa) to enable the connection. Both ends of the connection (caller and callee for example) must be able to send and receive UDP packets in order to establish and stabilise the VoIP connection, as well to be able to hear what both parties are saying.

When a UDP packet is dropped, it is essentially reaching one end of the connection, but not being sent back with SIP traffic to the other party, this could cause issues such as one way audio or certain/all phones not ringing when someone makes a call to your number.

More information about SIP can be found here: What is SIP?

In order to fix this issue, there is setting in any VoIP phone you will find called ‘Keep Alive Interval’ (the wording may be slightly different dependant on the device, but Keep Alive are the key words here!)

For example in a Grandstream GXP2130 you can head to: Settings > General Settings and you’ll find the setting here:

By changing this to 10 seconds (or more if you prefer, but 10 is the general consensus) the connection will check every 10 seconds to make sure that both ends of the connection are still there, hence stopping your calls from dropping.