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= Configuring a SPA504g for use with IP-Phone-C =

[https://asahi-net.jp/en/service/ipphone-c/ IP-Phone-C] is a VOIP service provided by [https://asahi-net.jp AsahiNet]. It is a rebranding of NTT's [https://ja.wikipedia.org/wiki/%E3%83%95%E3%83%AC%E3%83%83%E3%83%84#%E3%81%B2%E3%81%8B%E3%82%8A%E9%9B%BB%E8%A9%B1 Hikari] VOIP service, so these notes may be applicable to similar services from other resellers.

The information in this note should be applicable to other VOIP handsets as well, although their configuration details will vary.

== Subscriber Information ==
Configure an extension on the handset as follows:
 EXT > Display Name = <Your 050 Phone Number>
 EXT > User Id = <Your 050 Phone Number>
 EXT > Password = <Your Password>
 EXT > Use Auth Id = Yes
 EXT > Auth Id = <Your User Id>

== Audio Configuration ==
The IP-Phone-C SIP server only supports the G711u codec. Other codecs, or too many proposed codecs, may trigger a "488 Not Acceptable Here" response. Therefore configuration with just G711u is recommended.
 EXT > Preferred Codec = G711u
 EXT > Use Pref Codec Only = Yes

== NAT Configuration ==
If you are using NAT, enable keepalives:
 EXT > NAT Keep Alive Enable = Yes

== RTP Packet Size ==
The IP-Phone-C SIP server only accepts a SDP attribute a=ptime with the value 20. Any other value will result in a "488 Not Acceptable Here" response, incorporating a "307 Session description parameter not understood" warning. This is an extremely difficult issue to debug without a priori, and the engineer involved should be ashamed of themselves for not providing a more meaningful error response.

The SPA504g emits the correct value with the following configuration:
 SIP > RTP Packet Size = 0.02