Can I hook up a fax machine and an answering machine into a dsl line?

Yes you can. Most fax machines have a TAD (telephone answering device) function. When the fax is in TAD mode, it listens to the call answered by the external answering machine. If it detects a fax being sent, the fax machine takes over and starts communicating with the fax machine. If there is no fax tones, then the fax machine just lets the answering machine complete the call. On a DSL line, I think you just need to have the filter on the jack. Then connect the fax machine. Then connect the answering machine to the LINE / TAD jack on your fax machine. When you have your devices in this order, then the TAD function works. Consult your fax machine instructions to verify setup. You'll want your answering machine to answer calls quickly, like in 2 rings, so there's enough time for your fax machine to know if a fax is being sent. While this function exists, it can be "fragile". I had this setup for awhile, and it didn't receive faxes 100% of the time. If you send/receive faxes often, internet/email fax services are much more beneficial. It can actually cost less to use internet fax services sometimes.

Today, the use of fax machines is considered impractical because the installations and operating expenses is costly. The best alternative is the internet fax services, that only requires a computer and high-speed internet to operate. You just have to choose a reliable service provider to get high quality service.

Using a media gateway device you can even attach a GSM and PSTN network to your DSL line. As for the fax machine, I suggest you make use of an internet fax service.