Open standards is the way to go. When it comes to telephony, I will suggest it is better and more interoperable to implement an inbuilt click-to-call based on SIP rather than something suited to a single system such as what has been done with the "Asterisk Zimlet". You need to realize that different users/businesses run several different IPBXs that are not Asterisk-based and some even avoid Asterisk due to lack of scalability amongst other things. So it is a no-brainer to;
1. Rework the click-to-call feature as an inbuilt feature of Zimbra rather than a Zimlet
2. Rework the zimlet to initiate click-to-call based on the SIP format. That is, to send the called party's number as well as the calling party's number and the bridge to whatever IPBX the user may be using in a SIP format to initiate and bridge the call between both numbers.
With such an implementation, SIP will receive these credentials and do just that, and everyone's choice of PBX will be addressed rather than having a clunky "Asterisk-style" module. Asterisk users are stuck in the "Asterisk module" way of thinking rather than thinking open standard. This was the same problem with the Asterisk Skype module until Skype realized that if they wanted to reach farther they had no choice but to go open standard, so they now have SIP interop with
Skype for SIP Beta - Skype .
We are a 501(c)(3) technology non-profit (
Open Software Foundation - The Foundation for Information & Communications Technology.™) currently evaluating telephony/UC engines to derive a robust open source UC solution in-line with Zimbra (similar to and beyond how OCS augments Exchange) with a rich Ajax UI (perhaps YUI). We are not at all entertaining the idea of Asterisk because we need the system to be scalable and interoperate well enterprise-wise not just SMB, amongst other things (we certainly need Zimbra developers as well as others to join/work with us on this). Freeswitch seems more like what may fit this model because other engines seem to be a derivative of the very limited Asterisk.
Unless you only plan to reach a small group of users or small office rather than enterprise users open standard should be the default model of development and not some patch work to only fit a certain system.