reference: wikipedia on VOIP: 

"Communication on the IP network is 
perceived as less reliable in contrast to the circuit-switched public 
telephone network because it does not provide a network-based mechanism 
to ensure that data packets are not lost, and are delivered in 
sequential order. It is a best-effort network without fundamental 
Quality of Service (QoS) guarantees. Voice, and all other data, travels 
in packets over IP networks with fixed maximum capacity. This system may 
be more prone to data loss in the presence of congestion[a] than 
traditional circuit switched systems; a circuit switched system of 
insufficient capacity will refuse new connections while carrying the 
remainder without impairment, while the quality of real-time data such 
as telephone conversations on packet-switched networks degrades 
dramatically.[16] Therefore, VoIP implementations may face problems with 
latency, packet loss, and jitter.[16][17]"


Based on my own experience

I've seen VOIP have occasional dropouts.
I've seen copper be bombproof. 

When copper was laid, it was done with care,
and it was built to last. VOIP is tacked onto internet connections as an
afterthought. It can work, but it can certainly not work. Where
on the other hand, copper will always work. Even if the power or internet
goes down.

Verdict: VOIP or Copper is ok, but copper will always be more reliable.