Voice MPLS Network between 4-sites intermittant connectivity loss

I have 4-sites that are connected via Telepacific MPLS T1 Circuits which provide each site with a Voice VLAN and Data (Internet) VLAN
The data (Internet) seems to work fine but the private VLAN for voice seems to be intermittent and stops responding to pings however, when this happens the Avaya phone equipment is still able to dial across the Voice VLAN and (MPLS Circuit) using the H323 lines in the Phone System for voice calls.

The Telco provider is providing a Cisco2911 at each site with 2-Ethernet Connections and 2-gateways. x.x.x.1=Internet, x.x.x.2=Voice

I had the telco provider look into this issue but of course, the are unable to find anything as to why the Voice VLAN stops responding.

Any thoughts, ideas or suggestions as to what my next step should be or how should, recommendations?

Below is the site configuration foe each of the 4-sites:
SITE-1: 192.168.1.1 Data/Internet; 192.168.1.2 Voice

Site-2: 192.168.2.1 Data/Internet; 192.168.2.2 Voice

Site-3: 192.168.3.1 Data/Internet; 192.168.3.2 Voice

Site-4: 192.168.4.1 Data/Internet; 192.168.4.2 Voice