For use with OnSIP, this is a fatal problem with the Unidata phone.
OnSIP requires phones to be compliant with the SIP standard - SIP RFC 3261 (http://tools.ietf.org/html/rfc3261). Unidata has apparently acknowledged that their phone is not compliant with the SIP standard and they have confirmed that their phone cannot reliably place calls through a forking proxy (OnSIP calls are routed through a forking proxy). In detail, the phone cannot handle more than a one 'early dialog' as defined in the main SIP RFC 3261 - see Section 13 of the RFC for some details. As such, the phone cannot reliably complete calls where responses are received from multiple to destinations due to a forking proxy as required by the RFC. They should absolutely fix this if they claim the phone is compliant with the SIP standard RFC 3261.
Here are some relevant excerpts from the RFC:
13 Initiating a Session
When a user agent client desires to initiate a session (for example, audio, video, or a game), it formulates an INVITE request. The INVITE request asks a server to establish a session. This request may be forwarded by proxies, eventually arriving at one or more UAS that can potentially accept the invitation. These UASs will frequently need to query the user about whether to accept the invitation. After some time, those UASs can accept the invitation (meaning the session is to be established) by sending a 2xx response. If the invitation is not accepted, a 3xx, 4xx, 5xx or 6xx response is sent, depending on the reason for the rejection. Before sending a final response, the UAS can also send provisional responses (1xx) to advise the UAC of progress in contacting the called user.
126.96.36.199 1xx Responses
Zero, one or multiple provisional responses may arrive before one or more final responses are received. Provisional responses for an INVITE request can create "early dialogs". If a provisional response has a tag in the To field, and if the dialog ID of the response does not match an existing dialog, one is constructed using the procedures defined in Section 12.1.2.
188.8.131.52 2xx Responses
Multiple 2xx responses may arrive at the UAC for a single INVITE request due to a forking proxy. Each response is distinguished by the tag parameter in the To header field, and each represents a distinct dialog, with a distinct dialog identifier.
See our top business VoIP phone recommendations