Hi, does it work with a fax machine connected to it? the phone company mentioned "for some people he fax works, for some it doesn't". Does it make sense they can not tell me in advance if the fax will work or not? also, I have 256kb/second upload speed and 1MB/second download speed - is this sufficient for reasonable quality when using Voice over IP technology? THANKS.
Voice Over IP Technology for Home Phone – Will My Fax Machine Work?
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VoIP lines "can" support Fax transmissions successfully.
But, there are many factors and variables that will determine whether you will have success sending Fax over a VoIP line. This is why your VoIP service cannot guarantee successful Fax transmission and receptions... they can't control all the factors involved.
Everyone knows that Internet speed is a factor. But, most people have ample high-speed Internet to accomodate Fax. This is not actually the biggest problem with Fax over Internet (also called FoIP).
Just be sure to not surf or file download or email while sending or receiveing Faxes to reduce any data bottleneck issues. And, yes, set your Fax machine speed to low as possible. This will improve success rate.
Other Internet factors the user does not have control over are:
Packet Jitter, packets dropped, or excessive delays resulting in dropped packets.
Your local network setup can have influence on success of fax. If you have a separate Router after the modem and before your VoIP ATA adapter, the Router should have QOS enabled (if it has it). QOS will give VoIP and FoIP highest priority over other local network traffic. QOS can reduce some jitter and delay issues.
Your VoIP adapter plays a big role in Fax success. If not configured to be optimized for Fax, then your will likely not have success with Faxing. Your VoIP ATA must be set for a high-bandwidth Codec like G.711u and set in PassThrough mode. Because Fax signals are modulated with analog tones, those tones must be accurately replicated - this can only be done with a high-bandwidth codec (G.711u or PCMU) in PassThrough mode.
Success with Fax over Internet (FoIP) can be optimized by using T.38 Codec. The T.38 Codec is specifically designed for FoIP. Many VoIP adapters are now capable of supporting T.38 Codec. For T.38 codec usage to be most successful, your VoIP adapter and your VoIP carrier both need to be T.38 compatible.
So, as you can see there is not simple yes or no answer as to whether Fax will work over VoIP lines - many factors involved.