The problem having a fax machine on a the same line as your phone are.
1. You leave it switch on, some one phones and end up listening to your fax machine if you don't answer the phone. 2. Someone send you a fax without you knowing and you end up listening to a fax machine as it is likely you will answer the phone.
If you are going to use the fax on the same line as your phone I would suggest you make sure you know when you are going to receive a fax.
I think some fax machines may allow you to listen and then answer if it is a caller and not a fax.
If you are with BT you can have a second number on the same line click here click here Other providers may have a similar service.
I use this service in conjunction with this click here click here
Cheaper than a second line and works well. The question is would you really use the fax on a daily basis, (Ido because I work from home) but if faxing is a rare event I would connect it as and when needed.
Most fax machines will allow you to plug in a handset to answer the line if it is a voice call. You can alter the number of rings before it answers; so set it to answer after 6 rings that will give you time to answer. If you have an answer-phone then it gets complicated.