Why can't I make calls with USB-C to 3.5 adapter on iPhone?

You're running into a subtle but very real difference between audio passthrough and full audio routing, particularly for phone calls. What you're seeing is typical behavior with many USB-C to 3.5mm adapters—especially third-party ones that aren't MFi or Apple-certified. The issue isn't with your iPhone 16 Plus per se, but rather with how the adapter handles audio streams, particularly voice call audio, which is treated differently by iOS than music or media audio.

When you were using your iPhone 12 with a Lightning-to-3.5mm adapter, Apple’s own hardware or MFi-certified adapters supported CTIA-standard analog audio, and also allowed for proper routing of call audio (input and output). With USB-C, Apple has shifted to a more USB Audio Class 1.0 compliant interface, and not all USB-C adapters support the full feature set, including hands-free profile (HFP) which is required for phone calls. Many generic adapters will happily pass along music but will not support mic input or call audio at all.

Some things you can try:

• Use an Apple USB-C to 3.5mm adapter – This is key. Apple’s version is more likely to support full functionality including phone calls, since it adheres to the expected audio routing for iOS.
• Check for microphone support – Your current adapter might not support microphone input. If there's no mic passthrough, your phone won't detect a proper hands-free device and will default the call to internal speaker or another output.
• Bluetooth fallback – If your car has Bluetooth, iOS will route call audio over Bluetooth and media through the cable unless otherwise configured. That can get confusing and may be worth checking in Settings → Bluetooth → [Car Name] → tap the info “i” and see what’s enabled.
• Test with Voice Memos – Plug in the adapter and try recording a voice memo. If it doesn't pick up any sound, your adapter doesn't support mic input, which confirms the issue.