Softphone with Proxy?

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Rear one,
I look for myself here and with Google already since hours the wolf and have unfortunately still nothing suitable thing found to the following problem:
I have nen 1&1 DSL connection and a Voip Flat. Since I often am on business trip, I search a Softphone for my Lappi, which also also behind a Proxy functioned (like often in firm networks the case is).
The 1&1 Softphone does not make unfortunately the input possible of a Proxys.
Which would be beautiful, even if the Softphone still one event. attached camera would support and/or. if the opposite side sends a picture this indicates.

I found numerous left to Softphones, which make that possible allegedly. But everyone which I so far tested, went then somehow nevertheless not.

In hope to have surveyed, I am not grateful to anything for each tip.

Best greetings