Some interesting protocols exist (alongside tel) for mobile I wasn't aware off:
http://www.mobilexweb.com/blog/click-to-call-links-mobile-browsers

tell: vs callto: is apparently an ongoing battle:
http://stackoverflow.com/questions/1164004/how-to-mark-up-phone-numbers

Should we just remove the tel links all together and let real phones *cough* infer them ?