VPN detection is usually done on server side.
Diitalk simply has to check your IP and compare it to a list of IP ranges by common VPN providers. If the IP is in the IP range of a known VPN provider it blocks you. This is known as blacklisting.
Therefore the highest change to bypass such a VPN check is to use a custom server and install your own VPN on it. If the server's IP is not in a range used frequently for VPN your VPN may stay undetected.
However the service you are trying to use may use a different VPN detection technique, like whitelisting. The IP ranges from US telecomunication providers are known and one can simply check if the IP is within a range or not. In such a case any VPN provider or self-installed VPN will fail.