The Grandstream GXP2000 is an office SIP phone. It is fairly straightforward to setup via the phone's web interface. Below is a screenshot with highlighted options needed for it to register and work properly with SDF's VOIP system. Here are some items to note:
Grandstream HT801/HT802 are Analog Telephone Adapters which support analog phones, including older rotary dial models. Newer firmware versions allow pulse dial and high powered ringing. The HT802 supports two lines.
The Grandstream Handytone 286 is a simple analog telephone adapter. It can allow you to use any analog phone with the SDF VOIP service. It can be configured using the built-in web interface or through voice prompts by dialing *** on an analog phone.
You should also forward UDP port 5060 to the Handytone's IP address through your router. It may be a good idea to set the Handytone to a static IP address, which can be done on the Basic Settings tab. Don't forget when doing this to add all the relevent fields, including a proper DNS server, else the Handytone won't be able to resolve the sip.sdf.org address.
Android 2.3 and up seems to have a built-in SIP client. The screenshots below are from Android 4.3. This is tested with wifi data, and 3G/HSPA on tmo. Some carriers or specific android versions may disable SIP calling over cellular data.
When making a call from the dialpad, there does not seem to be a way to enter the @ sign. If "Use Internet calling" is set to "For all calls", then this is not an issue: just type the extension number and tap call..
Another way to make SIP calls is to add the SIP number (nnnn@sip.sdf.org) into the contacts, and select the number from the address book. Android seems to detect the @ sign and automatically switch to internet calls regardless of what the "internet calling" setting and the "Set as primary account" setting is set to.
Finally, Google Contacts also has the option of labeling a number as "Internet call" which will trigger SIP calling as well.
This tutorial is far from complete. Wanna make it better? Edit it!