The end customer is interested in the function of the total chain operator - hardware. Below some experience from ten months use of a Nokia E61 with some major European operators. All with the intention to convey information resulting in better software for VoIP.
The problem is that VoIP is not stable over a longer time period.
I am using two operators; Freecall belonging to a group called Betamax selling under various brand names, and Sipgate.
Freecall, just for outgoing calls
For a long time it worked OK. Since some months nothing happened when trying to call. Occasionally a call was fullfilled but most of the time nothing happened. It turned out that maybe up to 10 minutes after I hung up the other party got the call, when answering nothing in the other end. I was billed for the call.
In the settings of the E61 there are three choices for the transport protocol; Auto, UDP and TCP and the default setting is Auto. It turned out that changing from Auto to UDP solved the problem. After doing that it works again. Using Auto or UDP doesn' t matter for Sipgate.
Sipgate, also for incoming calls
Over these ten months Sipgate has been the most reliable operator. They also have excellent set up instructions, screen dumps from an E61 with my personal settings blended into the pictures. Since some months Sipgate started loosing the registration at the server. It can happen relatively quickly after registering, it can take hours. It happens more often during the time of the day when it is heavy traffic. I am using Freecall and Sipgate simultaneously. Only Sipgate is loosing registration, Freecall never looses registration.
Loosing registration stops incoming calls of course. In practical life it also stops calling out, it might take several minutes to register again and I do not want to wait several minutes to make a call. In reality it makes the service inoperable.
I have been looking at the communications between the E61 and the server, all communication is UDP:
When getting the registration with the server I get a lease for 5 minutes, the time until I have to register again. Just before the end of these 5 minutes the phone initiates a new registration. In the correspondence some packages are exchanged in each direction. Normally a new registration is granted within 0,2 seconds which corresponds to the round trip time to the server. Sometimes I can see that the phone is initiating the process but does not get a reply. The phone continues trying for one or several minutes - I have not been able to catch the process when it looses registration. I do not know after which time the phone is considering the connection dead and closes down. This is what is happening, once the connection is closed the phone doesn't try again. It seems that the problem is due to heavy traffic to/inside the server that gets congested sometimes and the E61 disconnect. I have not found any remedy for this problem. The E61 is treated differently by the server compared to the software client which can be downloaded. When using that a lease for 10 minutes is granted every time.
The old fashion phone network we are used to was designed to have an availability of 99,999 % or better, called the five nines. Seen over the 10 months I have used VoIP with the E61 I don't think I have one nine even. At least two operators are needed to have one working.