Short answer: Use G.711 Codec and turn off T.38 support in your Router.
If that still fails, try lower baud rates in your Fax machine.
In the case of German Telekom, T.38 isn't supported. Your router might only try T.38 if that option is on. G.711 has (one of) the best quality when it comes to the ISDN frequency spectrum.
Long answer:
You can still send faxes, even over IP-Based lines. As these may be lossy (drop packets, async packet receiving), transmitting faxes may actually fail if your connection is bad.
Using the proper Codec to encode the analog tones digitally, analog fax signals can still be sent to and received from true analog telefax machines and lines.
Also, some routers only try to transmit a fax via T.38 if that option is available. Try disabling T.38 support as well.
If the carrier won't support t.38 you'll have issues, most don't and act like you are crazy for asking. g.711 can work but will probably give you headaches.
Best solution is get an analog line for fax only or look into a carrier that offers t.38 (https://us.babytel.net/en/fax/t38fax/). If fax is important just get a POTS line for each fax machine and be done with it.
It's a fact that Fax machines don't and won't work over VoIP Phone system. The only challenge that exists is that While VoIP converts the audio signal into computer language(0s and 1s), it distorts the signals received from fax machine and it can't be understood on the other hand.
There's no way of fixing this issue, however you may follow tips to resolve the same:
By reducing the transmission rate i.e baud rate.
Enable "QoS" via router. Doing so, this will make the transmission understood on other hand.
Short answer: Use G.711 Codec and turn off T.38 support in your Router. If that still fails, try lower baud rates in your Fax machine.
In the case of German Telekom, T.38 isn't supported. Your router might only try T.38 if that option is on. G.711 has (one of) the best quality when it comes to the ISDN frequency spectrum.
Long answer:
You can still send faxes, even over IP-Based lines. As these may be lossy (drop packets, async packet receiving), transmitting faxes may actually fail if your connection is bad.
Using the proper Codec to encode the analog tones digitally, analog fax signals can still be sent to and received from true analog telefax machines and lines.
Also, some routers only try to transmit a fax via T.38 if that option is available. Try disabling T.38 support as well.
If the carrier won't support t.38 you'll have issues, most don't and act like you are crazy for asking. g.711 can work but will probably give you headaches.
Best solution is get an analog line for fax only or look into a carrier that offers t.38 (https://us.babytel.net/en/fax/t38fax/). If fax is important just get a POTS line for each fax machine and be done with it.
It's a fact that Fax machines don't and won't work over VoIP Phone system. The only challenge that exists is that While VoIP converts the audio signal into computer language(0s and 1s), it distorts the signals received from fax machine and it can't be understood on the other hand.
There's no way of fixing this issue, however you may follow tips to resolve the same:
Hope these tips helps you!!