phranque - 8:52 am on Oct 12, 2010 (gmt 0)
there is a tel: URI scheme for telephone numbers, which i have seen work well on mobile sites.
obviously this is going to be client-dependent, so if you're browsing on a computer that isn't connected to telephony through a voice modem or can route VOIP service through to the public network, then you basically have a useless link.
RFC 3966 - The tel URI for Telephone Numbers:
you also have to deal with the various issues regarding local, 7- vs 10-digit local, long-distance and international calling.