Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
It may be that the problem is at the other end. Does this problem show up with only one fax number? if so, that may explain it.
Try dialing your cell phone # ***** the Canon L80. See if the call goes through, and if when you pick up the call on your cell phone, you hear beep tones from the L80 fax.
If so, then you probably don't have a problem with either your L80 fax, nor your phone line.
But also test it other ways, if you like.
And, if the call *doesn't* go through, then you may have a problem with the phone line service, or with the local phone cabling to your fax-phone.
Did the phone line service you have, work before this with the L80 fax-phone? if not, if you changed your phone line service provider, the phone line service might be at fault... esp. if the phone service is digital (i.e. UVerse, Vonage, MagicJack, VoIP, etc.)
Let me know, and I will advise you further. Thanks.