[Sip] Comment on willis-sip-answeralert-00

Regarding the UAS procedures for handling the Answer-Mode value of "ManualReq", the draft indicates that the UAS SHOULD answer in manual mode, and the UAS SHOULD reject the request if it is unwilling or incapable of answering in manual mode.

However, there is a use case for Push to Talk (as it is being specified in OMA) that might justify changing these SHOULDs to SHALLs (similar to how the UAS procedures for handling AutoReq are currently defined).  The requesting user may not want their initial talk burst to be automatically played out from the called UA's speaker phone, due to privacy concerns of the calling user.  The common example is a mistress who does not want the call to be auto answered, for fear that the wife is present.  Another example is when the requesting user is concerned that he/she might be interupting the called user (i.e. calling user suspects that the called user is having lunch with a customer, or is in a meeting) and therefore wants to "politely" alert the user prior to the initial talk burst being played out, in order to giving the called user an opportunity to reject the call.  In each of these cases, the calling user's behavior is based upon a "guarantee" from the PoC service that thei!
 r initial talk burst will only be played out if the called user manually accepts the call.

It also would seem more logical to have the semantics of "ManualReq" be truly "manual mode required", rather than the current "manual mode strongly recommended".