Yes, Express Talk can support up to 6 incoming phone calls at a time.
However, the amount of incoming phone calls may be restricted by other factors (such as limits set by your VoIP provider). Often outbound calls are completely separate to inbound, and some providers will for example only give you 1 incoming channel/line but provide you with for example 3 outbound lines/channels. Many services are reluctant to give you several incoming channels especially if you're on a flat rate plan for the DID because they will get not benefits from allowing so many calls, in fact they may actually lose out from the costs of bandwidth and whatnot from those calls.
Also, are you using Axon in between? Or are you connecting ET directly to the VoIP provider? Is the Voicemail system on your end (e.g. IVM)? Or is it going to the Voicemail system from your provider (this will help establish whether the provider is limiting the calls, or if the problem lies internally).