To ensure good call quality you should make correct codec settings.
Codec is a special program, used for encoding or decoding digital media. If you look at your SIP phone settings you will find several codecs. Why so many and how properly use them?
1. The most common codec is G.711 (it is called in your phone alaw and ulaw, may be G711a and G711u). If you use hardware phone, or a computer, connected to wired Internet, may be this is all you need. But for mobile devices this is not the best solution. Because bandwidth needed for this codec is about 90Kbit/s one way.
2. There are many modern codecs, that provide good quality at lower bandwidth. The main rule is – both sides must support this codec.
3. So, for mobile calls use low bandwidth codecs. For example for my mobile calls I usually use SILK – NB codec (about 14Kbit/s). The quality of no worse than with conventional GSM call.
4. Unfortunately, the SIP world is too big, and many SIP providers does not support modern codecs. So alaw (G711a), ulaw (G711u) or GSM codecs are mandatory when calling outside sipmobile.org.
Example of setting codecs:
The phone will try to use codecs from up to down. So, it the other side supports Opus Narrowband (a good modern narrowband codec), it will be used during the call. Next candidate is Opus Wideband – good high quality codec, but may be not so good for mobile calls, because of bandwidth.
Next comes iLBC – narrow band codec, which is currently a part of WebRTC project. It is widely used in smartphones. So you have good chance, that it will be used.
This codec, used in mobile phones. Is is also widely used.
And the last one is G.711 ( a and u-law).
For calls to VoiceMail and PSTN we now support GSM, G.711 a-law and G.711 u-law codecs. So, one of them must be enabled in your phone.