I have had this happen to me many times. Get a call from a recruiter who leaves a message and also sends me an email with the job information.
9 times out of 10 when I actually talk to recruiters I have them send me the email with the job description any way so they saved me a step.
After talking to the recruiter or receiving an email I will review the position. If after reviewing the position I find I have no interest, I will normally shoot them an email back thanking them for their time and why the position would not be a fit (This accomplishes 2 things says yes I am alive and this is what would should send me if you want me to change jobs). If the position is one I would like to follow up on I will also communicate via both methods. Depending on the time of day/my availability I will email back after reading the description that I would like to talk further about the position. When I have more time to talk and ask questions about the position I will call to follow up.
If I know it will be several hours to the next day before I can call them I always email. If I decide I am going to call them back at the next break I have I will forego the email and just call. Of course if I get their VM I will leave a message and send the email.
As far as an unwritten rule, I will normally go with one contact method over the other depending on how
fast I want to get a response (read desperately unemployed I am). If they have called and emailed I feel they have said either method is now fine. I may send an email back and then follow up with a phone call, but eventually if I want to pursue the position I will need to talk to the recruiter in person or over the phone.