Etiquette tips for dealing with a shared home office phone line?

I rent office space from a friend’s office in her home. We share a land line phone. Some of the people who call me assume she is my assistant (she is not, we don’t work for the same clients) and try to leave lengthy messages with her which is not appropriate—not to mention that the messages don’t always get to me. My friend has recently begun to refer people to my work cell phone which is fine. My friend’s gripe currently is that these people tend to call during the evening and weekend and she is stuck taking calls for me during non-office hours which is annoying to her. My question is this: Is it appropriate to note in my email signature that lengthy or urgent messages should be left on my cell? I want to say this: Please note that this is a shared home office line. If you do not reach me and have a lengthy or urgent message, please use my cell # below.**)
Is that TMI for an email signature? I don’t want to remove the land line # from my signature because I am trying to resist the urge to do all of my calls from a cell phone. I don’t need any brain tumors, thank you!