What to Look for in a SIP Softphone

A SIP softphone turns a computer, phone or tablet into an extension on your PBX or an endpoint on your provider's network. The category is crowded, and feature lists are long. Most of the differences that matter in daily use come down to about a dozen questions. This post walks through them, in roughly the order you should ask them, and notes where SIPdia fits, including where it does not yet.

Which platforms does it run on?

Start here, because it rules out most candidates immediately. List every device your team will use: Mac, Windows PC, Linux workstation, iPhone, iPad, Android phone. Then check which softphones cover all of them. Mixed environments often end up with two different apps, one for desktops and one for mobile, which is fine as long as both work with your PBX.

Check minimum OS versions too. An app that needs the latest release can leave older devices behind.

SIPdia runs on macOS 14 and later, and on iOS and iPadOS 17 and later. There is no Windows, Linux or Android version at the moment.

Which transports does it support?

SIP can run over UDP, TCP or TLS. UDP is the traditional default and works with almost every PBX. TCP helps with large messages and with some NAT situations. TLS encrypts the signaling and authenticates the server, and is what you want whenever traffic crosses a network you do not control.

Look for all three. Also check how the app validates TLS certificates. Some accept any certificate, which defeats the purpose of TLS. Others require a certificate from a public certificate authority. The stricter behaviour is safer but means a PBX with a self-signed certificate must be reached over a VPN instead.

SIPdia offers UDP, TCP and TLS, performs DNS SRV lookup when no port is given, and requires a public-CA certificate for TLS.

Does it encrypt the audio?

TLS protects signaling only. Audio runs over RTP on separate ports, and plain RTP can be recorded by anyone on the path. SRTP, defined in RFC 3711, fixes that. A good softphone lets you turn SRTP off, offer it optionally, or require it, because PBX support varies.

SIPdia's Encrypt audio setting has exactly those three modes: Off, Optional and Required.

Which codecs does it offer?

Codecs determine audio quality and bandwidth. The practical minimum is G.711 in both ulaw and alaw variants, because it is the most widely supported VoIP codec and a common baseline on PBXs and trunks. Whether a call actually uses it still depends on the codecs the server or trunk allows and on SDP offer/answer negotiation. Opus is a strong addition: it adapts to available bandwidth and sounds better on poor connections. G.722 gives wideband audio on many desk phones. G.729 saves bandwidth but was patent-encumbered for years and is still often missing or paid.

Match the list against what your PBX and trunk provider offer. A codec the server does not support is useless, and if the two sides share no codec, the call fails or has no audio.

SIPdia supports Opus, G.711 ulaw and G.711 alaw. It does not include G.722 or G.729.

Can it hold several accounts?

If you have more than one extension or provider, you want them registered at the same time, with a clear way to pick which one places an outgoing call. Check whether multiple accounts are included or gated behind a paid plan, and whether per-account settings such as feature codes are supported.

SIPdia registers several accounts at once and shows a From picker when dialing. The free plan allows one account; SIPdia Premium removes that limit.

Transfer, hold and multiple calls

These are the controls you use dozens of times a day. Look for:

• Hold and resume, with a clear indication of which call is on hold.
• Blind transfer, where you send the call to another number and drop out.
• Attended transfer, where you speak to the target first and then connect the two parties.
• Several simultaneous calls with easy switching between them.

Try these on a real PBX before committing. Transfer in particular depends on both the app and the server behaving correctly.

SIPdia supports hold, blind transfer, attended transfer between two open calls, and several concurrent calls.

Call recording and its legal side

Recording is useful for notes, training and disputes. Check where recordings are stored, in what format, and whether the feature is included or paid. Then check the law. Many places require that at least one party consents to recording; some require that all parties do. A softphone cannot tell the other side that you are recording unless you do it yourself, so build that into your process.

SIPdia records locally to WAV files on the device as part of SIPdia Premium. Each call can be recorded once, and the app shows a notice before recording that the other party is not informed.

How does it handle NAT?

Most devices sit behind a router doing network address translation. Without help, the PBX receives media addresses that are only valid inside your network, and you get one-way or no audio. Look for STUN support, ICE support and the ability to set an outbound proxy. Good NAT handling in the app, combined with correct NAT settings on the PBX, solves the large majority of audio problems.

SIPdia supports STUN, ICE and a separate outbound proxy setting.

Can it receive calls when it is in the background?

This is the question most people skip and later regret. On a desktop, an app that keeps running in the background can keep its SIP registration alive and ring when a call arrives, as long as the machine is awake. On iOS and modern mobile platforms, the operating system suspends apps shortly after they leave the foreground. To receive a call, the app must be woken by a push notification sent through the platform's push service. Apple provides PushKit for exactly this, and RFC 8599 describes how a SIP server tells a push gateway to deliver it.

The catch is that someone has to run the push gateway. It needs a server component that knows your device token and talks to Apple's or Google's push service. Some providers run one for their own apps. Many PBX setups do not have one, and on iOS an app without push support cannot ring once the system has suspended it, no matter how well it is written.

SIPdia does not have push notifications for incoming calls at the moment. On iOS the app must be open, and it unregisters about 20 seconds after leaving the foreground. On macOS it keeps running when the window is closed and stays registered while the Mac is awake, but a sleeping Mac misses calls. Call forwarding on no answer, set through PBX feature codes, is the usual workaround. Read why some softphones must stay open for the full explanation.

How is it licensed and paid for?

Softphones range from free open source, through freemium with a paid tier, to per-seat subscriptions. Understand what is gated, whether a purchase covers all your devices, and whether the vendor is likely to keep updating the app. A one-time purchase from a vendor that stops updating is a poor deal once the OS moves on.

SIPdia is free with one account and core calling, and SIPdia Premium adds recording, additional accounts, call forwarding and Do Not Disturb. A Premium purchase through Apple covers iPhone, iPad and Mac on the same Apple Account.

What does it do with your data?

Read the privacy policy. Find out whether the app includes analytics or advertising SDKs, whether call metadata leaves your device, and where the SIP password is stored. A softphone handles sensitive information and should be judged accordingly.

SIPdia contains no analytics, ads or tracking, and stores the SIP password in the Apple Keychain. It does require signing in with an Apple or Google account.

A short checklist

1. Runs on every device you use.
2. UDP, TCP and TLS, with sensible certificate validation.
3. SRTP with an optional and a required mode.
4. G.711 plus Opus at minimum, matched to your PBX.
5. Multiple accounts if you need them.
6. Hold, blind transfer, attended transfer.
7. Recording, if needed, with a plan for consent.
8. STUN and ICE for NAT.
9. Push notifications if the phone must ring when locked, and a server that can send them.
10. Clear licensing and an honest privacy policy.

Next steps

Compare these points against the SIPdia feature list and read what a SIP softphone is if you are new to the topic. When you have narrowed the field, free vs paid SIP softphones looks at the pricing side.