I think dealing with phone calls is easily the biggest challenge working from home with kids. It's so hard because you never know (particularly with babies) when they will cry or whine or need something urgently.

I don't know if they have this in Spain, but in the States there are a few services that mimic office phone systems. You can even have different lines, music for when people are on hold, etc. I think you can even check your phone messages on email. Try doing a search for virtual voice mail or something like that. That way, you could turn your phone ringer off at bad times and people will at least feel like they had contact with an office. Perhaps you could also state your email address on your phone message as well. So many people use email a lot that they might try that, and I find it's much easier to deal with email at off times than a phone conversation.