How to unhold a specific call using Action URI

Via Action URI we can use the key=F_HOLD command to unhold a call, this works fine when there is only one call.

When there are two calls on hold how can I tell the phone via action URI to unhold the call I want to unhold?

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And if there are 3 held calls the issue is even more complex. The action URL/URI system needs to support passing the CSTA_ID so exact call control is possible.