Not sure you guys are following me here... I've never seen an analog/digital phone system (NOT VoIP) that used an RJ-45 connector on their phones. VoIP is an entirely different beast that uses existing network cabling.
If they're upgrading from analog/digital to VoIP - there's no need to run new cable - they use their existing network infrastructure - VoIP phone plugs into existing jack, PC plugs in to VoIP phone. Done. Just have to have PoE switches, VLAN's set up, QoS, etc... etc... and PoE isn't even necessary if your phones will take an external power supply or an inline PoE power injector.
If they're doing VoIP from the get-go (i.e., new construction), they're using an existing data drop at the user's desk - there's NO need to run a separate network connection dedicated VoIP, so long as their switches are configured properly for it. Some customers insist on it - but that's because either they have network security requirements that call for it, their network infrastructure is not ready for it, or they don't know what they're doing. In most cases, VoIP phone systems integrate with their networks.
What I'm talking about is an existing facility with a analog/digital phone system and a separate data network. And for that, you have to use RJ-11 jacks for voice - otherwise you're sending 48 volts from the PBX (or telco if they're dedicated lines for fax, alarm systems, etc...) into a NIC and creating the potential for damage to the customer's equipment - can't plug an RJ-45 into an RJ-11, and there's a reason why they went that route when the standards were created. If you're punching your voice cable down at the frame into 66-blocks, BIX, 110, etc... they're being cross-connected to the phone system, and not into your network infrastructure.
Yes, I've seen customers insist on terminating everything to regular 110-patch panels with RJ-45's and using CAT5 patch cables to cross-connect everything, but it's very expensive, looks like crap, a nightmare to maintain - especially when you're getting into 100's or 1,000's of phones), and in the end creates more work. In the real world - you terminate all your voice to 66-blocks (or BIX - I prefer 66 blocks) and use inexpensive cross-connect wire to link everything up.