Hi naumburg,
Re: it immediately hands over the call to my iphone. why is that?
Have you checked out the problem with the carrier / call-provider ?
Wondering if you immediately raise your wrist when you hear the call, and want to answer it.
(If the iPhone is nearby, iPhone and Watch may both ring, and both be available to answer.)
Could the Watch be on "Silent" ?
or
Would this article help ?
Eg: When a call comes in, the caller is identified and you’re asked if you want to answer the call.
Announce calls with Siri on Apple Watch – Apple Support
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Re: is that by (some stupid) design?
This Apple Support article explains how it is designed to work.
Answer phone calls on Apple Watch – Apple Support
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Re: or is something off with my device?
Wondering which settings are in use, on iPhone, on Watch, and with Handoff.
Eg:
Check the cellular set up on Apple Watch:
(To turn your cellular connection on or off, swipe up from the watch face to open Control Center.
Tap the green cellular button , then turn Cellular on or off.)
Set up cellular on Apple Watch - Apple Support
Hand off tasks from Apple Watch – Apple Support
says: "Handoff is on by default."
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More info:
Use Dual SIM iPhone with Apple Watch cellular models – Apple Support
Eg: If you set up multiple cellular plans using an iPhone with Dual SIM,
you can add multiple lines to your Apple Watch with cellular,
then choose the one your watch uses when it connects to cellular networks.
Note: Each iPhone cellular plan must be provided by a supported carrier and must support Apple Watch cellular.
Also:
Apple Watch - Official Apple Support
You might want to check out the problem with the Apple Watch Support Staff
All the best :-)