The adapter is not really locked or unlocked. If you gain access to the admin password you can program the ATA adapter to any other service.
SR has a better trial offer: 31 days vs. VTs 14 days. If you get phones with the service, you can keep the phones as a free gift ($50 value) even if you cancel service. There is also no shipping cost, no setup cost, no cancellation fee. VT charges $10 shipping cost, plus you have to pick up shipping cost back to them, so a total of $20 down the drain if you don't like VT. Also, after the trial is over, SR has no cancellation fee whatsover and you get a full pro-rated refund if at any point you wish to cancel. VT keeps 50% of your money, a huge risk in case they have major service problems and you wish to try someone else.
Also, SR has a lot more features than VT and they are running special deals once a month that bring service cost down to $8.25 per month or $8.29 per month, respectively (via either a $199 for 2 yr. deal or $99 for 1 yr. deal.) Add also that SR gives you a free signature # and also a $3 international call allowance.