docs(sip): state that SIP presence and PIDF-LO are not supported

fern/products/platform/pages/calling/voice/SIP/index.mdx

@@ -18,6 +18,8 @@ SIP is fundamental to modern Voice over IP (VoIP) communications and enables fea
 - Presence information
 - Multimedia distribution
 
+These are capabilities of the protocol in general. See [Presence and location support](#presence-and-location-support) for which of them SignalWire supports.
+
 Getting started with SIP involves understanding a few key components:
 - SIP endpoints (phones, softphones, or applications)
 - SIP servers (proxies, registrars)
@@ -62,6 +64,8 @@ SIP phone systems enable you to perform a wide range of functions that a traditi
 - Enabling instant messaging, multimedia conferences, user presence information, and E911 calls for emergencies.
 - Bridging different sets of equipment and software by negotiating common capabilities before the calls start, akin to two people who speak different languages finding a way to communicate. SIP has the benefit of working with diverse vendors and setups as long as they share even a fraction of similar capabilities.
 
+These are functions of the protocol, and support for them varies by provider. See [Presence and location support](#presence-and-location-support) for the location, availability, and presence items on this list.
+
 #### Affordability
 
 As we alluded to earlier, a big benefit of alternating to a SIP trunk is price-oriented. We had mentioned how you save on long-range calls and international calls due to every call being viewed as a local call through SIP. However, there are other areas that a SIP trunk saves money in, including:
@@ -82,6 +86,16 @@ SIP calling consolidates your entire communication network into a handful of dev
 
 Most SIP providers offer wideband audio or HD voice with a higher sample rate of telephone audio, giving your phone higher fidelity audio than other systems.
 
+## Presence and location support
+
+SIP presence and PIDF-LO location objects are two SIP features that other providers sometimes bundle with a SIP trunk. SignalWire handles both differently: presence isn't supported, and emergency location comes from an address you register rather than from the call.
+
+SIP presence is the SUBSCRIBE/NOTIFY event package that lets one SIP device watch another's status, with state carried in PIDF (Presence Information Data Format) bodies. Busy lamp field (BLF) keys and online/offline indicators in desk phones and softphones depend on it. SignalWire doesn't accept presence subscriptions, so devices registered with SIP Credentials can't watch each other's status. The Dashboard shows when a SIP Credential last registered, but there is no API or webhook for registration state.
+
+PIDF-LO (Presence Information Data Format Location Object, RFC 4119 and RFC 5491) is how some networks attach a per-call location to a 911 call, carried on the INVITE through a `Geolocation` header. On SignalWire, the location a dispatcher receives comes from the emergency address you register and assign to the phone number. See [E911](/docs/platform/e911) to set one up, and update the assigned address when a device moves.
+
+To request either capability, contact [SignalWire support](https://support.signalwire.com).
+
 ## Using a softphone with SignalWire [#softphone-configuration]
 
 Any SIP-compatible softphone (such as Zoiper, Bria, Linphone, MicroSIP, or 3CX) can be used with a SignalWire SIP Credential.