Switchvox it is a commercialproduct not supported on this forum, but it looks that on the provisioning phone get a dial plan who instruct the phone to dial such number
If the phone’s auto-dialing 800, then there’s probably some latent config on the phone or otherwise being sent to the phone instructing it to do some form of PLAR. The phone is the only thing that decides to make a call, Switchvox (Asterisk) just waits for the endpoint to send an INVITE. So, have a look at any autodial/plar settings for your handset and make sure that the dial / digit plan being used on the phone isn’t such that 800 might be dialed automagically.
I’m not Switchvox support. The above posting from @ambiorixg12 still stands.